CHAPTER 5

Catalytic Cracking andReforming

1 IntroductionThe processes of feedstock recycling of plastic wastes considered in this chapterare based on contact of the polymer with a catalyst which promotes its cleavage.In fact, plastic degradation proceeds in most cases by a combination of catalyticand thermal effects which cannot be isolated. As was described in Chapter 3, theuse of catalysts is also usual in chemolysis processes of plastic depolymerization.However, there are two main differences between catalytic cracking andchemolysis: there is no chemical agent incorporated to react directly with thepolymer in catalytic cracking methods, and the products derived from thepolymer decomposition are not usually the starting monomers.

Compared to the simple cleavage of the polymer by thermal effects, catalyticcracking has a number of advantages:

The polymer molecules start to break down in the presence of catalysts atconsiderably lower temperatures than in thermal decomposition. Asignificant catalytic conversion of polyoleflns into volatile products hasbeen detected at temperatures as low as 200 0C, compared with the valueof 400 0C which is necessary in the thermal degradation of PE and PP toobserve the formation of the first gases. As a consequence, catalytictreatments of plastic materials are usually carried out at low tempera-tures, in contrast with the range of 500-8000C, typical for thermalcracking and pyrolysis.When compared at the same temperature, catalytic cracking of polymersproceeds faster than thermal degradation, i.e. with lower activationenergy. At temperatures of about 400 0C, the first volatile products areobserved after only a few minutes of contact with the catalyst.The products derived from the catalytic cracking of plastics are of higherquality than those obtained by thermal decomposition. Thus, the presenceof a high proportion of branched, cyclic and aromatic structures in theoils produced lead to properties very similar to those of commercial

gasolines. Moreover, the product distribution can be varied andcontrolled by the selection of a suitable catalyst and modification of itsproperties.

All these factors point out the great potential of catalytic cracking for theconversion of polymeric wastes into valuable products. However, this methodalso suffers from a number of drawbacks and problems, which are still notcompletely solved. The catalysts are deactivated with time by the deposition ofcarbonaceous residues and poisons present in the raw waste stream, such as Cland N compounds. Moreover, the inorganic compounds contained in theplastic wastes tend to remain with the catalysts, hindering their recovery andreuse. For these reasons, catalytic cracking is mainly applied to polyolefinicwastes of relatively high purity, a number of pretreatments being required toremove all those components which may negatively affect the catalyst. Otherdifficulties arise from the high viscosity of the molten plastic, which hinders itsflow through conventional fixed bed reactors. These problems are largelyavoided when the catalytic conversion is combined with a simple thermaltreatment, aimed at reducing the viscosity of the mixture and enabling theseparation of unwanted components. In this case, the catalytic step consistsreally of a reforming of the products directly formed by thermal degradation ofthe polymers.

In the following sections of this chapter, the catalytic conversion of individualplastics (polyethylene, polypropylene and polystyrene) is first reviewed, fol-lowed by a description of the processes developed for the catalytic cracking ofplastic and rubber mixtures. Finally, methods based on a combination ofthermal and catalytic treatments are considered. However, taking into accountthat the key factor in the catalytic conversion of plastic wastes is the catalystitself, we will first describe the main properties of the most widely used catalyticsystems for the degradation of polymers.

2 Types and Properties of Polymer Cracking CatalystsA wide variety of catalysts have been found effective in promoting thedecomposition of plastic materials: Friedel-Crafts catalysts, acidic and basicsolids, bifunctional solids, etc. Friedel-Crafts systems, mainly A1C13/HC1, wereinitially used as acid catalysts but they have now been replaced in mostprocesses by solids with acid properties due to the corrosion and environmentalproblems they cause.

The most common catalysts used in plastic cracking are acidic solids, mainlyalumina, amorphous silica-alumina and zeolites. These materials are thecatalysts typically used in the petroleum processing and petrochemical indus-tries. They have very different textural and acid properties, which directlydetermine their catalytic activity and product selectivity. Thus, while the acidityof alumina is of Lewis type, both Bronsted and Lewis acid sites may be presentin amorphous silica-alumina and zeolites. This is an important factor because

the initiation step of polymer catalytic degradation depends on the type of acidsite: by proton addition over Bronsted sites and by hydride abstraction overLewis sites, which leads to different cracking pathways. The concentration ofacid sites in both amorphous silica-alumina and zeolites can be controlled bychanging the Si/Al ratio, because the acid sites are generated by Al species.Moreover, the Al content of the catalyst usually influences the strength of theacid sites. Stronger sites favour the cracking reactions, although they may alsopromote undesired reactions, such as coke deposition which can cause catalystdeactivation.

Alumina and amorphous silica-alumina are usually mesoporous materialswith a wide distribution of pore sizes. The surface area, pore size and porevolume of alumina and amorphous silica-alumina depend greatly on thepreparation method, hence their textural properties can be controlled to acertain extent by changing the synthesis conditions. These parameters are alsohighly relevant in determining the catalytic properties of these materials.

On the contrary, zeolites are by definition microporous crystalline silicoalu-minates. They have a perfectly defined crystalline structure based on the linkagebetween SiO4 and AlO4" tetrahedra through oxygen bridges. The presence ofpores with sizes below 1.0 nm in zeolitic structures allows different molecules toenter, diffuse and react within them. At present there are over 100 known zeolitestructure types,1 some of which occur naturally, although those having the mostsignificant catalytic applications are synthetic materials. Zeolites are classifiedaccording to their pore size (small, medium and large pore zeolites), the numberof channel systems (unidimensional, bidimensional and tridimensional porezeolites), and Al content (low, medium and high silica/alumina ratio zeolites).

Depending on the topology and the preparation method, the Al content ofzeolites can be varied over a wide range, from a Si/Al ratio of unity to negligibleamounts of aluminium.2 The acid form of zeolites is obtained when the negativecharge associated with the framework Al species is balanced by protons. The Alcontent can be varied by synthesis or post-synthesis methods and it is usuallyaccompanied by significant changes in the zeolite acid strength. Moreover, bothBronsted and Lewis acid sites can be present in zeolites, depending on theirstructure and the Si/Al ratio. Protons can be ion exchanged by other metalcations, which affects both the acid properties and the effective pore size.Moreover, zeolites with basic catalytic properties can be generated by ionexchange with cations such as Cs.

The features of the zeolite channel systems are also key factors in explainingtheir catalytic properties. The possibility of discriminating between reactantsand products according to their molecular size compared with the zeolite porediameter is a widely reported phenomena, known as shape selectivity.3 Com-pared with amorphous silica-alumina, zeolitic catalysts exhibit a number ofadvantages: a higher acid strength related to their crystalline structure,narrower distribution of pore sizes, higher stability under thermal and hydro-thermal conditions, lower rate of coke deposition, etc. The most commonzeolites are also those which have been most extensively used for the catalyticcracking of polymers: X, Y, ZSM-5, mordenite, etc. Figure 5.1 shows the

B)ZEOLITEY

Figure 5.1 Pore structure of different zeolites: (A) ZSM-5, (B) Y.

structure of zeolites Y and ZSM-5, and the main features of several zeolites aresummarized in Table 5.1.

As a consequence of all these properties, zeolites can be considered to beexceptional catalysts which have replaced amorphous solids in many applica-tions. However, for the catalytic cracking of polymeric wastes, zeolites may bedisadvantageous due to the steric and diffusional problems that polymermolecules may have in accessing the zeolite micropores. These drawbacks canbe overcome with the use of zeolitic catalysts with very small crystal size and,therefore, with a high proportion of external surface area which is not subjectedto steric hindrances for the conversion of bulky substrates.

Other interesting solids for the catalytic degradation of polymeric wastes arethe various silica-based mesophases which have recently been discovered.4'5

These materials are characterized by the presence of ordered and regular poresystems and high surface areas, typically over 1000 m2 g~l. The most commonmember of this family is MCM-41, which has a hexagonal array of uniformpores with diameters that can be tailored in the range 1.5-10 nm by varying thesynthesis conditions. These mesoporous materials can be prepared with a wide

A) ZEOLITE ZSM-5

Table 5.1 Main properties of various zeolites

Zeolite Structure Si/Al Ratio Pore size (nm)

ZSM-5 MFI 10-1000 0.53 x 0.56,0.51 x 0.55Y FAU 1.5-3* 0.74Beta BEA 8-1000 0.76 x 0.64,0.55 x 0.55Mordenite MOR 5* 0.65 x 0.7

* The Si/Al ratio of zeolites Y and mordenite can be increased by post-synthesistreatments.

range of framework compositions and exhibit properties of both conventionalzeolites and amorphous silica-alumina: uniformity of pore sizes, regular poreordering, amorphous nature of the pore walls, presence of both Lewis andBronsted acid sites of weak and medium strength, etc. The combination of highsurface area with uniform mesopores in MCM-41 are the reasons which supportthe remarkable catalytic properties that this material has recently shown for theconversion of polyolefins.6'7

In addition to silica- and alumina-based solids, activated carbon impregnatedwith transition metals and sulfated zirconia have also been tested in the catalyticcracking of organic polymers. Activated carbons are microporous solids with agraphitic-like structure and large surface areas. The incorporation of transitionmetals on activated carbon leads to the generation of hydrogenating/dehydro-genating active sites, which promotes hydrogen transfer reactions during plasticdecomposition. Sulfated zirconia is known as a superacid solid, i.e. its acidstrength is greater than that of 100% H2SO4. Thus, the Hammet acidity ofsulfated zirconia is —16.04, appreciably higher than the value of —11.94corresponding to 100% H2SO4. Sulfated zirconia is therefore commonly usedas a catalyst in reactions requiring strong acid sites.

3 Catalytic Conversion of Individual PlasticsPolyethylene

Most of the studies reported on the catalytic cracking of plastics use PE asstarting material because it is the main polymer in plastic wastes. The firstworks appeared in the 1970s,8 mostly based on the use of Friedel-Craftscatalysts.

Ivanova et al? have described in detail the mechanism of PE degradation overAlCl3-based catalysts. Compared with thermal decomposition, the catalyticconversion of PE at 400 0C leads to higher conversions with significant changesin the product distribution, because the catalyst also promotes secondaryisomerization reactions. While the thermal decomposition of PE takes placevia a radical mechanism, the catalytic cracking over acid solids proceedsthrough carbocationic species. According to Ivanova et al.,9 the initiation stepinvolves the formation of carbocations through hydride abstraction from thepolymeric chains by the H + [AlCl3-OH-] catalyst with release of H2. This attack

may take place at random positions in the polymer chains or, if side branchesare present, selectively at tertiary carbons. A preferential degradation may alsooccur at double bond defects, in this case by proton addition. The carbocationsformed are isomerized to yield more stable tertiary cationic species, which inturn undergo /?-scission or elimination of the side chain. Other reactions whichoccur in the system are intermolecular and intramolecular transfer processes,similar to radical mechanisms.

Beltrame et al.10 have compared the thermal degradation of PE with catalyticcracking over amorphous silica-alumina and zeolite catalysts. Figure 5.2 showsthe TGA corresponding to the PE degradation alone and in the presence of10 wt% of zeolite HY (protonic form of zeolite Y). While thermal conversionof PE into volatile compounds starts at 400 0C, the catalytic decompositioncauses a significant weight loss at temperatures below 200 0C and the polymer isalmost completely converted before reaching 4000C. The authors have alsodetermined the activation energy of the PE degradation from TGA data, asignificant reduction being observed in the presence of the catalysts. In contrastwith the high activation energy found for the thermal degradation(273 kJ mol"1), catalytic degradation over zeolites HY and REY (rare earthexchanged form of zeolite Y) led to values of 57.3 and 41.OkJmOl"1. Asignificant decrease was also observed for silica-alumina materials, with activa-tion energies around 120 kJ mol~~l. This effect was less significant when silica oralumina were used as catalysts, activation energies of 193.1 and 181.8 kJ mol ~ l,respectively, being calculated. According to these results, the decrease in theactivation energy of PE degradation appears to be correlated with the acidstrength of the catalysts.

weigh

t los

$(%)

Pi • ZHY PC

T(0C)Figure 5.2 TG analysis in nitrogen of PE alone and a PE+HY mixture (\0 wt% of

HY)W

(Reprinted from Polym. Degrad. Stab., 26, P.L. Beltrame, P. Carniti, G.Audisio and F. Bertini, page 209. © 1989, with permission from ElsevierScience)

LIVE GRAPHClick here to view

The presence of these catalysts also leads to meaningful variations in theproduct distribution. Figure 5.3 compares the GC analysis of the oil fractionobtained by Mordi et al.u in the degradation of LDPE alone and in the presenceof a ZSM-5 catalyst at 3500C. The products of the thermal degradation weredistributed over a wide range of carbon atom numbers, the peaks appearing asdoublets or even triplets, which indicates the presence of the correspondinglinear alkane, alkene and diene for each atom carbon number. In contrast, theproducts of the catalytic oil are mainly below Ci5, and several peaks are presentfor each atom carbon number, indicating the presence of multiple isomers,mainly alkanes and aromatics, whereas the amount of a-olefins is negligible.

The mechanism and products of the PE catalytic decomposition overamorphous silica-alumina catalysts has been investigated by Ishihara eta/.,12'13 using a batch reactor under nitrogen atmosphere. Four fractions wererecovered after the reaction: gases, liquids, degraded oligomer and degradedpolymer. The last two fractions were obtained from the residue remaining in thereactor by partial dissolution in xylene, the degraded polymer being thesolubilized fraction. The formation of gases and oils was detected at tempera-tures as low as 2200C, although operating with high catalyst concentration(catalyst/polymer mass ratio, C/P = 1). The authors propose a sequentialmechanism to explain the catalytic PE degradation:

polymer -* degraded polymer -> degraded oligomer -• liquids -» gases

A)B)

Figure 5.3 GC analysis of the oils derived from LDPE degradation at 3500C:(A) thermal conversion, (B) catalytic conversion over ZSM-5. Aromaticcomponents identified: o-xylene (8a), p + m-xylene (8b), naphthalene(JOa), n-methylbenzene (JOb), methyl-naphthalene (JJa), solvent (S).(Reprinted from J. Anal. Appl PyroL, 29, R.C. Mordi, R. Fields and J.Dwyer, page 45. © 1994, with permission from Elsevier Science)

Ret. time (tnin)R«t. tint* (min)

The gases produced contained over 50 mol% of isobutane, and minoramounts of isobutene, propylene, propane, isopentane, etc. Interestingly, theliquid degraded oligomer and degraded polymer fractions were free of olefmiccompounds, in contrast to the high concentration of double bonds typicallypresent in the products of thermal degradation. The 13C-NMR spectrum of thedegraded oligomer fraction, shown in Figure 5.4, indicates that these oligomerscontain a large number of short branches, most of them C6 or less in length. Themajor types include short linear chains, ranging from methyl to pentyl, andsome branched alkyl chains. The authors found a relationship between themolecular weight reduction and the branching frequency of degraded oligomersand polymer. Figure 5.5 shows the concentration of branched methyl groups,excluding those at the end of the backbone, versus the average molecularweight. It is concluded that the number of side chains increases linearly with thereduction in the molecular weight of the degraded polymer. The high degree ofbranching is due to the isomerization activity of the amorphous silica-aluminacatalyst, which promotes conversion of secondary carbocations into tertiaryones, rather than jS-scission. It is proposed that gases are formed selectivelyfrom the liquid components having the highest branching frequency.

Ishihara et al.14 have also studied the catalytic degradation of PE overamorphous silica-alumina in a continuous flow fixed bed reactor. Carefulmeasurements of the temperature gradients in the reactor showed that some ofthe PE degradation takes place by thermal cracking in the high temperaturezones located prior to the catalytic fixed bed. Therefore, when the molten

ppm from TMSFigure 5.4 13C-NMR spectrum of the oligomer fraction obtained by PE catalytic

degradation over amorphous silica-alumina.n

(Reprinted from / . Appl. Polym. Sci.y 38, Y. Ishihara, H. Nambu, T.Ikemura and T. Takesue, page 1491. © 1989, with permission from ElsevierScience)

Mn (x104 )Figure 5.5 Relationship between the molecular weight reduction and the branching

concentration during the PE degradation over amorphous silica-alumina.13

(Reprinted from J. Appl. Polym. Sci., 38, Y. Ishihara, H. Nambu, T.Ikemura and T. Takesue, page 1491. © 1989, with permission from ElsevierScience)

polymer is contacted with the catalyst, it is really made up of thermallydegraded oligomers. Under these conditions, very high yields of gaseousproducts were observed with an overall polymer conversion of about 90%, inspite of the low residence time in the catalyst bed (~20 s). Gas yields of over70 wt% were obtained in the temperature range 400-475 0C. In this system, theprevious thermal cracking contributes not only to a decrease in the molecularweight, but also to a generation of olefin groups in the oligomers, which favoursthe subsequent catalytic decomposition by proton addition from the catalysts toboth internal and chain end double bonds.

The effect of PE properties on its catalytic degradation over amorphoussilica-alumina catalysts has recently been studied by Uddin et al.15 in a batchreactor at 4300C. Four different types of PE were used: high density PE(HDPE), low density PE (LDPE), linear low density PE (LLDPE), and cross-linked PE (XLPE). Compared with thermal degradation, in all cases thecatalytic conversion was 3-4 times faster. In both thermal and catalyticdegradation, HDPE and XLPE were converted at a slower rate, indicating thegreater difficulty of cracking these polymers, probably as a consequence of thelower concentration of branches in their polymer chains. In the catalyticexperiments, conversions of about 90% were obtained and no waxy productswere formed. The main products were liquids with yields of about 80%. Onlyslight changes were detected in the product distribution corresponding to thecatalytic cracking of the four PE types.

PE catalytic degradation over zeolite NaY was also investigated by Ishiharaet al.16 in a batch reactor. The authors propose a sequential mechanism similar

CH

3/1

00

0C

H2

to that previously described over amorphous silica-alumina. One of the majordifferences found is the higher proportion of isopentane present in the gasesproduced over the zeolite. The oligomer fraction was composed mainly ofsaturated structures with a high number of methyl branches, and some olefinicsignals were detected by 1H-NMR in the liquid fraction. It is proposed thatisobutane and isopentane are generated by decomposition of C9 species presentin the liquid fraction.

Lin et al.11 have investigated, using TG measurements, the deactivation of thezeolite USY (ultrastable zeolite Y) by coke deposition during the degradation ofHDPE. The cracking activity of previously coked zeolite samples was deter-mined by successive reactions with fresh polymer. A significant decrease inactivity was observed, which led to a progressive shift of the HDPE degradationtowards higher temperatures. The activity was found to decline exponentiallywith the coke content. Thus, half of the initial catalytic activity is lost for a cokecontent of 4.24 wt% in the catalyst. Nevertheless, most of the initial activity canbe recovered by calcination of the catalyst in air, as can be seen in Figure 5.6showing the TG analysis of HDPE and USY mixtures through several reaction-regeneration cycles.

The catalytic properties, including resistance to deactivation, of differentzeolites (HY, H-mordenite and ZSM-5) and amorphous silica-alumina haverecently been compared by Uemichi et al.ls in a fixed bed reactor. The yields ofthe different products obtained at 4500C are illustrated in Figure 5.7, whereas

Wei

ght

Rem

aini

ng /

%

Temperature /0CFigure 5.6 TG analysis of HDPE + USYmixtures through several reaction-regeneration

cycles.l

(Reprinted from Thermochim. Ada, 294, Y.-H. Lin, P.N. Sharratt, A.A.Garforth and J. Dwyer, page 45. © 1997, with permission from ElsevierScience)

RO

Rl

R2

R3

R4

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Figure 5.7 Yields of products obtained in the catalytic degradation of PE at 450 0C in afixed bed reactor over different catalysts:18 ZSM-5, HY, H-mordenite (HM)and amorphous silica-alumina (SA).(Reprinted with permission from Y. Uemichi, M. Hattori, T. Itoh, J.Nakamura and M. Sugioka, Ind. Eng. Chem. Res., 37, 867. © 1998 ACS)

the variation in coke deposition with time on stream is shown in Figure 5.8.ZSM-5 was very active for the conversion of PE, mainly into gaseous products.A high initial activity was also observed over the HY zeolite, but a large amountof coke is formed, which leads to a very rapid deactivation of this catalyst withincreasing time on stream. The fast deactivation of HY is confirmed by the cokecontent of about 20 wt% detected in this sample. The presence of large cavitiesin the structure of zeolite Y favours the formation of polyaromatic species,which are coke precursors. In contrast, the main products derived from PEdegradation over amorphous silica-alumina and H-mordenite are liquid hydro-carbons, although a high proportion of wax was also produced on the latter,showing the lower activity of this zeolite. Moreover, H-mordenite was veryrapidly deactivated, probably due to the unidimensionality of its channelsystem, which is easily blocked by coke deposition. The best catalyst wasZSM-5 zeolite, in terms of both activity and deactivation. The liquid yield overZSM-5 remained constant, and coke deposition on this zeolite levelled off after150 min. Likewise, amorphous silica-alumina resulted in a stable catalyst inspite of the rapid coke deposition which occurs. In this case, the large pores ofthe amorphous silica-alumina (2-8 nm) means that this material is little affectedby coke deposition, at least in the range of time on stream investigated. Theliquid produced over the ZSM-5 zeolite has a boiling point in the range ofcommercial gasolines. GC-RON (gas chromatography-research octanenumber) measurements indicate that this liquid can be considered a highquality gasoline with research octane numbers between 103 and 112, as a

Yie

ld (

wt%

)

Gas Liquid Wax Coke

ZSM-5 HY HM SA

Time on stream (min)

Figure 5.8 Evolution of coke formation with time on stream during the catalytic degrada-tion of PE at 450 0C in a fixed bed reactor over different catalysts:18 ZSM-5,HY, H-mordenite (HM) and amorphous silica-alumina (SA).(Reprinted with permission from Y. Uemichi, M. Hattori, T. Itoh, J.Nakamura and M. Sugioka, Ind. Eng. Chem. Res., 37, 867. © 1998 ACS)

consequence of its high aromatic content. In contrast, the liquid obtained overthe amorphous silica-alumina sample had a lower RON number (about 85).

Lin and White19 have also compared the activity of different acid catalysts(amorphous silica-alumina, ZSM-5 zeolite and sulfated zirconia) for PEdegradation based on thermogravimetry-mass spectrometry (TG-MS)measurements. The rate of PE catalytic cracking and the reduction observed inthe activation energy were directly related to the acid strength of the catalyst,being in the following order: sulfated zirconia > ZSM-5 > amorphous silica-alumina. For the three catalysts investigated, PE degradation proceeds in twostages at different temperatures. The first maximum in the formation rate ofvolatile products may result from catalyst protons attacking defective doublebonds of the polymer, whereas the second maximum at higher temperature mayreflect the acid-catalysed cracking of the -CH2- polymer backbone. The maincomponents of the gases produced were olefins (propylene and isobutene),which is in contrast with the high proportion of isoalkanes found in previousstudies of PE catalytic conversion. The authors propose that olefins aretransformed into alkanes by protonation of the double bond followed byhydrogen abstraction from the polymer, but these reactions do not take placein TGA tests, probably due to the rapid removal of the volatile products fromthe reaction medium. ZSM-5 zeolite produced the largest fraction of aromatics,which was related to the restricted channel volume of this zeolite which favoursoligomerization reactions of olefins to form small alkyl aromatics. This conclu-

Cok

e (g

/g-c

atal

yst)

ZSM-5SAHYHM

sion is supported by the fact that aromatics are detected only after the formationof significant quantities of alkenes.

In a recent work the catalytic degradation of both HDPE and LDPE wasinvestigated by Aguado et al.1 in a batch reactor at 400 0C over three differentcatalysts: the mesoporous material MCM-41, zeolite ZSM-5, and an amor-phous silica-alumina sample. For both polymers, the activity order found wasas follows: ZSM-5 > MCM-41 » amorphous silica-alumina. The higher con-versions obtained over the zeolite were assigned to its stronger acidity, whereasthe superior activity of MCM-41 compared to the silica-alumina was related tothe large surface area of MCM-41 (over 1000 m2 g"1). Similarly to the thermalcracking reactivity of these two polymers, the catalytic cracking of LDPE wasfaster than HDPE over the three catalysts investigated, probably due to thehigher degree of branching in its backbone. Figure 5.9 compares the productdistribution obtained in the LDPE and HDPE degradation over MCM-41 andZSM-5. Polyolefin cracking over the zeolite leads to a high proportion ofgaseous hydrocarbons rich in olefins and a liquid fraction in the range ofgasolines (C5-C12) with a high aromatic content. Over MCM-41 less olefinicgases are generated, while, in addition to a gasoline fraction, middle distillateswith hydrocarbons in the range Cj3-C22 are also produced. This productdistribution is in agreement with the pore size of the catalysts, middle distillatesonly being formed over the mesoporous material. Comparing the two types ofPE, it is seen that with both catalysts higher amounts of oils are obtained in theLDPE cracking.

Sakata et al.20 have also studied polyethylene degradation over a mesoporoussilica catalyst. The material used, called KFS-16, is closely related to MCM-41,although it is prepared by a different method starting from a layered silicate(kanemite). One of the most interesting observations in this work is the PEcracking activity exhibited by KFS-16 in spite of the absence of acid sites (it is acompletely silica-based material). Figure 5.10 shows the cumulative volume ofliquid products obtained in the thermal and catalytic cracking of PE in a batch

SELE

CTIVI

TY (W

T %)

Figure 5.9 Product distribution obtained in the catalytic conversion of HDPE and LDPEat 400 0C in a batch reactor over ZSM-5 zeolite and MCM-41 catalysts.1

HDPE

LDPE

HOPE

LDPE

MCM-41 ZSM-5

C1-C4C5-C12

C13-C22

Lapse time / min

Figure 5.10 Cumulative volume of liquids produced in the thermal and catalytic degrada-tion of HDPE at 430 0C in a batch reactor.20 Catalysts: ZSM-5, KFS-16 andamorphous silica-alumina (SA-I, SA-2).(Reprinted from J. Anal Appl. PyroL, 43, Y. Sakata, M.A. Uddin, A.Muto, Y. Kanada, K. Koizumi and K. Murata, page 15. © 1997, withpermission from Elsevier Science)

reactor at 430 0C over samples of amorphous silica-alumina, ZSM-5 zeolite andKFS-16. It can be seen that the rate of PE conversion over KFS-16 is similar tothat of the amorphous silica-alumina and clearly superior to that of ZSM-5.The authors explain the activity of non-acidic KFS-16 by the stabilization of theradicals produced in the PE thermal degradation within the catalyst pores,which accelerates the degradation reactions. The absence of acid sites in KFS-16also caused a slower deactivation of this material compared with the acidcatalysts. Figure 5.11 shows the liquid products obtained in successive crackingexperiments over the same KFS-16 sample. Slight changes are observed fromone experiment to another, the activity being in all cases superior to that ofthermal degradation. Regeneration of the catalyst by calcination in air at 600 0Callowed most of the initial activity to be recovered.

A completely different catalytic system for PE conversion in a fixed bedreactor has been studied by Uemichi et al.21 The catalyst consisted of activatedcarbon impregnated with different transition metals (Pt, Fe, Mo, Zn, Co, Ni andCu), which led to a bifunctional material with both cracking and dehydrogena-tion/hydrogenation activity. In all cases, high conversions were obtained andthe main products were linear alkanes and aromatics, with little formation of

Cu

mu

lati

ve v

olu

me

/ m

lT

emperature / 0C

Thermal

SA-ISA-2ZSM-5KFS-16B

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Lapse time / minFigure 5.11 Cumulative volume of liquids produced in the thermal and catalytic degrada-

tion of HDPE at 430 0C in a batch reactor20 Catalysts: KFS-16 reused up tofour times and then regenerated by calcination at 600 0C.(Reprinted from J. Anal. AppL PyroL, 43, Y. Sakata, M.A. Uddin, A.Muto, Y. Kanada, K. Koizumi and K. Murata, page 15. © 1997, withpermission from Elsevier Science)

isoalkanes and alkenes. The major effect of the metal incorporation on theactivated carbon was to increase the selectivity towards aromatics and todecrease the formation of w-alkanes. The addition of the metals also caused acertain reduction in the overall conversion due to a higher coke deposition. Thebest yields in aromatics were obtained over Pt, Fe and Mo supported onactivated carbon. The authors also investigated the effect of the space time (theratio of feed mass flow to catalyst weight), a maximum in the aromatic yieldbeing observed for all the catalysts (Figure 5.12). At low space times, thearomatic selectivity tends to zero, which shows that aromatic hydrocarbons areformed from decomposed fragments but not directly from PE itself. At higherspace times, aromatics are converted through dealkylation and coking reac-tions, which explains the observed decrease in the aromatic yield. The activatedcarbon in this system is not a simple metal support, as can be concluded fromthe high activity obtained with this material alone. Moreover, impregnation ofPt and Fe over alumina and amorphous silica-alumina led to catalysts withlower yields of alkanes and aromatics, as they promote the formation of alkenesand isoalkenes. It is proposed that the initial hydrogen abstraction from the

Cum

ulat

ive

volu

me

/ ml

Thermal

1st run2nd run3rd run4th run

Regenerated

0

12

3

4

5

1

2 3,45

0

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W/F (g cat min/g PE)

Figure 5.12 Relationship between contact time ( W/F) and aromatic yield in the catalyticdegradation of PE in a fixed bed reactor over metal-supported activatedcarbons:21 O C, # Mo/C, 0 Ni/C, 0 Zn/C, A Fe/C, A Cu/C, D Pt/C, •Co/C.(Reprinted from 7. ^na/. /*/?/?/. PyroL, 14, Y. Uemichi, Y. Makino and T.Kanazuka, page 331. © 1989, with permission from Elsevier Science)

polymer takes place on the activated carbon surface, and the resulting hydrogenatoms migrate to the metal sites where they are easily desorbed.

In contrast with the great research effort put into the preparation, character-ization and testing of different catalytic systems for the conversion of plastics,only a few studies have been published aimed at the design and operation offeasible catalytic reactors. Thus, most of the studies previously described usesimple batch or fixed bed reactors, in spite of the problems associated with thehigh viscosity and low heat transfer of the molten plastics. Some authors havealso investigated the catalytic degradation of PE in fluidized bed reactors.22'23

Scott et al.22 used an activated carbon bed for the degradation of LLDPE,obtaining large amounts of gaseous products, probably as a result of the hightemperatures of operation (500-785 0C).

Likewise, Sharratt et alP have recently developed a fluidized bed system forthe catalytic conversion of plastics. The catalyst is pelletized to give particleswith sizes in the range 75-180 /mi and is fluidized by passing through anitrogen stream. Once the reaction temperature is reached, the polymer isadded as particles with diameters between 75 and 250 /mi. After melting, thepolymer wets the catalyst surface, being pulled into the catalyst macropores bycapillary action. The reactor works with low polymer/catalyst ratios between0.2 and 1.0. This system has been used for the catalytic degradation of HDPEover ZSM-5 zeolite in the temperature range 290-4300C. The main products

Yield of aromatics (wt%)

LIVE GRAPHClick here to view

obtained were gaseous hydrocarbons with yields up to 70wt%, whichincreased with the reaction temperature. At 3600C, a conversion of over 90%was obtained after just 15min of reaction. The gases were rich in C3-C4

olefins, whereas C5 olefins were the predominant hydrocarbons in the liquidfraction. The aromatic content of the latter depended strongly on the reactiontemperature, with a maximum at 390 0C. These results show the possibility ofobtaining high yields of C3-C5 olefins, with a variety of applications inchemical synthesis, by catalytic cracking at temperatures about 35O0C. Aswas described in Chapter 4, high conversion of PE into olefinic gases is onlyachieved by thermal degradation when working at high temperatures, usuallyabove 700 °C.

Another alternative for the feedstock recycling of plastic wastes by catalyticdegradation consists of the direct feeding of plastics dissolved in gas oil fractionsto FCC (fluidized catalytic cracking) refinery units. Using this method, theconversion of PE blended with a vacuum gas oil (VGO) has been investigated ina fixed bed reactor at 5100C over a commercial FCC catalyst.24 It wasconcluded that the amount of PE in the blend is critical. The cracking of ablend containing 5 wt% of HDPE produced little or no gasoline, but only gasand coke. However, as the HDPE concentration was increased to 10 wt%, asignificant amount of gasoline was produced. Further increases in the PEcontent of the raw mixture yield blends which are too viscous for feeding to theFCC unit.

Polypropylene

The catalytic conversion of PP has been investigated using catalysts, reactorsand conditions very similar to those described above for PE. A decrease in theactivation energy of PP degradation in the presence of catalysts has also beenobserved by Audisio et al.25 based on TGA measurements, although this effectwas not as remarkable as in the case of PE. Compared with the activationenergy corresponding to the thermal cracking of PP (122.TkJmOl"1), thecatalytic conversion over HY and REY led to values of 108.9 and117.7 kJ mol"1. A greater reduction was observed over an amorphous silica-alumina catalyst, with an activation energy of 99.2IcJmOr1. The authorspropose that this relatively small effect of the catalyst in PP degradation isprobably associated with the presence of a methyl substituent in the polymerchain, and therefore of a high concentration of tertiary carbons, which makesthermal cracking easier, weakening the catalyst effect.

The mechanism of PP degradation over amorphous silica-alumina catalystshas been investigated by Ishihara et al.u in a batch reactor. The authorspropose a sequential pathway, as in the case of PE degradation. Under the samereaction conditions, PP is more rapidly converted than PE. Thus at 280 0C, 1 hof reaction, and catalyst/polymer ratio = 1, a PP conversion of 43% is obtainedwhile just 20% of PE is degraded into volatile products. The higher reactivity ofPP is due to its higher branching frequency, which favours cracking reactions.

Sakata et al.26 have also studied the degradation of PP over amorphous

silica-alumina catalysts. At 38O°C the products obtained consisted of 68.8%liquid hydrocarbons, 24.8% gases and 6.4% residue. The gases formed weremainly butene and propylene, while the liquids contained a complex mixture ofhydrocarbons in the C5-Cj6 range. Compared with the results obtained in thethermal cracking and in two-step (thermal-catalytic) treatments, the higher PPdegradation rate was obtained by direct contact between the polymer and thecatalyst. This result confirms that the solid acid catalyst in contact with themelted plastic promotes the polymer degradation, participating in the earlierstages of the cracking mechanism.

A detailed characterization of the products formed in the catalytic degrada-tion of PP over amorphous silica-alumina and CaX zeolite has been performedby Uemichi et al.,21 using a fixed bed reactor at temperatures between 470 and526 0C. For both catalysts the gases produced were mainly isobutene andisobutane, a higher proportion of the alkane being obtained over the zeolitecatalyst. Similarly, the liquid fraction consisted mainly of isoalkanes andalkenes, and minor amounts of linear alkanes and aromatics. Most of thebranching was due to the presence of side methyl groups. The products formedover CaX were more saturated, with lower amounts of alkenes and aromatics.Compared with thermal degradation, the products derived from the catalyticconversion contained greater quantities of aromatics and monomethyl-branched hydrocarbons, especially those with branching at the odd number.CaX was deactivated faster than the amorphous silica-alumina, due to theincreased coke deposition that takes place over the zeolite.

Zhao et al.2S have studied PP catalytic degradation over different zeolites bymeans of TGA measurements. The following order of activity was observed:zeolite Y > mordenite > zeolite L. PP conversion over HY led to hydrocarbonsconcentrated in the range C4-C9, and some new compounds with cyclicstructures were found compared to the thermal degradation.

The limitations of microporous catalysts for the conversion of PP haverecently been pointed out by Aguado et al.,1 by comparing the activity andproduct distribution obtained over ZSM-5 zeolite, MCM-41 and amorphoussilica-alumina. Figure 5.13 shows the conversion and the product distributionobtained in a batch reactor. It is remarkable that the ZSM-5 zeolite led to aconversion just slightly superior to that of the thermal degradation, in contrastwith the high activity obtained over this catalyst in the conversion of bothHDPE and LDPE. On the contrary, a significant activity was observed over thesilica-alumina catalyst, while MCM-41 led to a total conversion of the polymerinto mainly gasoline and middle distillate fractions. The low activity exhibitedby the ZSM-5 sample in the PP conversion has been related to the sterichindrance for the access of PP oligomers into the zeolite channel system, due tothe presence of side methyl groups on half of the backbone carbons. Thisconclusion was supported by the results of the molecular simulation of theadsorption of PP oligomers into the ZSM-5 structure, which showed that theycannot be accommodated within the pores and cavities of this zeolite.

Other possibilities for the catalytic conversion of PP over zeolites are to usezeolitic structures with a larger pore size or to increase the proportion of

Figure 5.14 Conversion obtained in the thermal and PP catalytic cracking at 400 0C in abatch reactor.29

external surface area through a reduction in the zeolite crystal size. Figure 5.14compares the activity obtained during the degradation of PP over differentzeolites.29 Increasing the pore size (zeolites USY and Beta) causes a certainimprovement in the catalytic activity, although it is still very low compared tothat of mesoporous MCM-41, which indicates that strong steric and/ordiffusional constraints hinder the PP conversion. However, a significantincrease in the conversion is obtained when using ZSM-5 with very smallcrystals as catalyst.30 In this case, the initial PP cracking takes place over the

The

rmal

ZSM

-5

Bet

a

USY

ZSM

-5na

nocr

yst.

MC

M-4

1

PP

con

v. (

%)

Figure 5.13 Conversion and product distribution obtained in PP catalytic cracking at4000C in a batch reactor over ZSM-5, MCM-41, and amorphous silica-alumina?

external surface area to yield small fragments that can enter the zeolite pores,where they are converted through further cracking, isomerization and aroma-tization reactions.

In addition to amorphous silica-alumina and zeolites, activated carbons havealso been used as catalysts for PP cracking. Thus, Nakamura and Fujimoto31

have studied the degradation of PP over a catalyst of Fe supported on activatedcarbon in a batch reactor at temperatures between 380 and 4000C. It isproposed that the cracking reaction is initiated by free radicals present on theactivated carbon surface through the abstraction of hydrogen from the tertiarycarbons of the PP chains. The addition of Fe on activated carbon caused a slightincrease in the activity compared to the support alone. However, incorporationof small amounts of CS2 or H2S led to a significant increase in the catalyticactivity of the Fe/activated carbon system and to a reduction in the productionof gases. The authors propose that hydrogen from H2S is abstracted by theradicals generated by thermal degradation of PP to form a stable hydrocarbonand an HS* radical, which avoids the consecutive cracking of the hydrocarbonradical and the formation of overcracked products (gases). As a consequence,the main products obtained by PP degradation with this system are naphtha,kerosene and gas oil fractions.

Polystyrene

In contrast with PE and PP, thermal degradation of PS takes place at relativelylow temperatures with a high yield of the raw monomer. Accordingly, in moststudies on the catalytic decomposition of PS, the polymer is simultaneouslydegraded by both radical and ionic pathways.

Audisio et al?1 have investigated the degradation of PS over differentcatalysts: alumina, silica, amorphous silica-alumina, and zeolites HY andREY. From TGA measurements, the activation energy of the thermal PScracking (277 kJ mol"1) was compared with that of the catalytic process. Inthis case, amorphous silica-alumina samples caused the greatest reduction inthe activation energy, to 45.6 kJ mol"1, followed by the zeolites with valuesof 96.1 and 115.SkJmOl"1 over HY and REY, respectively. Moreover,amorphous silica-alumina catalysts allowed the degradation temperature tobe reduced to about 2700C. While the major product of the thermaldegradation of PS is styrene, only minor amounts of the monomer wereobtained in the catalytic experiments, the main products being benzene,ethylbenzene, a-methylstyrene, toluene, isopropylbenzene and indane com-pounds. The authors propose that the catalytic degradation is initiated byproton addition to PS, which generates ionic species with the positive chargeon the benzylic substituent of the polymer chain. Subsequent jS-scission,isomerization and cyclization reactions lead to the formation of the differentaromatic products.

However, other authors have proposed that the primary product of the PScatalytic cracking is styrene, as in thermal cracking, which is further convertedinto ethylbenzene, toluene, benzene, etc. on the acid sites of the catalyst. De Ia

Conversion (wt.-%)

Figure 5.15 Selectivities of benzene (O), toluene (D), ethylbenzene (A), and crackingproducts (V) as a function of conversion at 5500C in the catalytic treatmentof PS (dashed line and open symbols) and styrene (filled line and closedsymbols) over a commercial FCC catalyst.(Reprinted with permission from G. de Ia Puente, J.M. Arandes and U A .Sedran, Ind. Eng. Chem. Res., 36, 4530. © 1997 ACS)

Puente et al.33 have studied the conversion of PS dissolved in benzene in afluidized bed reactor over a commercial FCC catalyst. The product distributionobtained in the catalytic degradation of PS was compared to that obtained instyrene conversion (Figure 5.15). The same relationship between the conversionand the selectivity towards the different products was observed in both PS andstyrene catalytic conversion at 5500C, suggesting that styrene is also theprimary product in the catalytic PS cracking. The authors proposed a mechan-ism to explain the formation of the main products from styrene.

A rapid deactivation of acid catalysts has been observed by Uemichi et al.ls

during PS cracking over HY, H-mordenite, ZSM-5 and silica-alumina, which isrelated to the high concentration of aromatic hydrocarbons. Likewise, Serranoet al.34 have observed that, depending on the reaction conditions, catalyticdegradation of PS may lead to lower conversions than a simple thermaltreatment. This is due to the existence of crosslinking reactions between thepolymer chains, which are promoted by the acid sites of the catalysts and yield aresidue of low reactivity. The extent of crosslinking versus cracking increaseswith the acid strength of the catalyst. Thus, very low PS conversions wereobtained in a batch reactor over ZSM-5 zeolite. Taking into account the poresize of this zeolite and the cross-sectional diameter of the PS molecules with sidebenzylic groups, it is concluded that crosslinking takes place over the externalzeolite surface.

Sele

ctiv

ity (%

)

4 Catalytic Conversion of Plastic Mixtures andRubber Wastes

This section describes the different processes that have been patented for thecatalytic conversion of plastic mixtures without any previous thermal treat-ment. In many cases, the authors claim that the process is also successful in thedegradation of rubber wastes or plastic and rubber mixtures.

While many studies have been carried out aimed at the feedstock recyclingof rubber wastes by pyrolysis and hydrogenation processes (see Chapters 5 and7), little information is found on the catalytic cracking and reforming ofrubber alone. Larsen35 has disclosed that waste rubber, such as used tyres,can be degraded in the presence of molten salt catalysts with properties asLewis acids, such as zinc chloride, tin chloride and antimony iodide. Thedecomposition proceeds at temperatures between 380 and 5000C to yieldgases, oil and a residue, in proportions similar to those obtained by simplethermal decomposition.

Wingfield et al.36 have developed a process for the catalytic treatment ofrubber and plastic waste by reaction in the presence of Zn and Cu salts(chlorides or carbonates). Examples are provided for the conversion ofautomobile shredder waste, containing a complex mixture of polymers(rubber, polyurethane, polyester, PP, PVC, ABS, etc.), at 550 °C to producechar and oil fractions, the latter containing about 0.5 wt% of S and N. Inanother patent,37 these authors disclose that plastic and rubber wastes can alsobe degraded in the presence of basic salt catalysts, such as sodium carbonate.Likewise, Platz38 has reported the catalytic conversion of plastics and rubber bycontact with a molten MgCl2/AlCl3 catalyst, whereas Butcher39 has reported onthe degradation of polymeric materials over molten mixtures of a basic salt(NaOH or KOH) and a Cu source, mainly metallic Cu and CuO.

Processes involving the use of solid acid catalysts have also been patented.According to Chen and Yan,40 plastic and/or rubber wastes are first subjected toa size reduction step, followed by separation of any metals present and washingto remove any non-plastic material such as paper, labels, etc. Subsequently, thepolymer wastes are dissolved or dispersed in a petroleum oil, with a high contentof polycyclic aromatic compounds at 300 0C, and catalytically transformed inan FCC reactor at temperatures of about 500 0C. Details are given for theconversion of different wastes: used whole tyres, PE bags and PS foam.

Saito and Nanba41 have developed several processes for the catalytic conver-sion of plastic wastes, one of them being shown in Figure 5.16. The plasticscraps are fed together with the catalyst to a first reaction vessel by means of ascrew placed above it. The catalyst/plastic ratio is typically in the range 0.05-0.1. In this reactor the plastics are melted and subjected to catalytic crackingunder agitation at temperatures between 400 and 470 0C. The volatile productsleaving the first vessel can be separated into fractions or alternatively they maypass through a second catalytic reactor (fixed bed) to further modify the productdistribution. The use of zeolitic catalysts is recommended. In the first reactor,the spent catalyst can be continuously removed through a screw feeder located

Figure 5.16 Schematic diagram of an apparatus for the thermal and/or catalytic degrada-tion of plastic scraps.41

(© K. Saito and M. Nanba, US Patent 4 584421, 1986)

in the bottom of the vessel and regenerated by treatment at 500 0C in a separateunit.

The catalytic degradation of a number of polymer mixtures commonly foundin plastic wastes has been reported by Evans and Chum,42 using two-stepprocesses at different temperatures in order to isolate the decomposition of eachpolymer and to avoid mixing of the corresponding degradation products.Thus, a mixture of nylon-6 and PP is first degraded at 293 0C over aKOH/alumina catalyst, which leads to the recovery of caprolactam from thedecomposition of nylon-6, and then it is subjected to a temperature of 397 0C torecover the products derived from the PP degradation. Similar treatments aredescribed for the catalytic conversion of other polymer mixtures: PET +HDPE, PET+ PS+ PE, PVC+ PU, polyphenylene oxide (PPO) + PS,PC + ABS, etc. It is proposed that when PVC is present in the startingmixture, the HCl evolved may catalyse the decomposition of the otherpolymers.

5 Conversion of Plastics by a Combination of Thermaland Catalytic Treatments

Two-step processes based on the combination of a previous thermal treatmentand a subsequent catalytic conversion, are widely used to facilitate the plasticflow and its mixing and contact with the catalyst. These two treatments can becarried out in two zones of the same reactor, in two different reactors of thesame plant, or even in two different plants.

Ohkita et al.43 have used a two-zone reactor for the catalytic degradation ofPE at 400 0C over amorphous silica-alumina and ZSM-5 zeolite. The polyolefin

is placed in the bottom of a stainless steel reactor, whereas the catalyst is placedin the middle part of the tube, so that the vapour generated by the thermaldecomposition of PE passes through the catalyst bed. Conversion over ZSM-5led to a higher proportion of gases, mainly C3 and C4, compared to amorphoussilica-alumina, whereas the oils were rich in aromatic hydrocarbons. Figure5.17 illustrates a GC analysis of the oils produced over this zeolite, withassignments of the peaks corresponding to aromatic compounds. Over amor-phous silica-alumina, the concentration of aromatics in the oils were observedto increase with the concentration of acid sites, which can be controlled byvarying the aluminium content of the catalyst.

A two-zone reactor has also been used for the conversion of PS at 350 0C overboth acid and basic catalysts.44 Considerable amounts of benzene and ethyl-benzene were formed over acid catalysts such as ZSM-5 and silica-alumina,showing that the styrene produced in the thermal step is further converted in thecatalyst bed by cracking and hydrogenation. However, with basic catalysts thefraction of styrene in the oils increased, values up to 75 wt% being observed.Figure 5.18 illustrates the yields of styrene monomer and dimer produced overthe different catalysts investigated. The degradation was faster over basicoxides, mainly with BaO and K2O, compared to the acid catalysts, which isassigned to the higher conversion rate of vaporized PS fragments and oligomerson the basic sites. It is proposed that PS degradation on solid bases is initiatedthrough the formation of carboanions by proton abstraction from PS on thebasic sites.

Figure 5.17 GC-MS analysis of the oils produced by thermal-catalytic PE degradation at4000C in a two-zone reactor with assignments of the peaks corresponding toaromatic hydrocarbons*3

(Reprinted with permission from H. Ohkita, R. Nishiyama, Y. Tochihara,T. Mizushima, N. Kakuta, Y. Morioka, A. Ueno, Y. Namiki, S. Tanifuji,H. Katoh, H. Sunazuka, R. Nakayama and T. Kuroyanagi, Ind. Eng.Chem. Res., 32, 3112. © 1993 ACS)

CatalystFigure 5.18 Yields of styrene monomer and dimer obtained in the thermal-catalytic

degradation of PS at 3500C in a two-zone reactor over various metal oxidecatalysts.44

(Reprinted with permission from Z. Zhang, T. Hirose, S. Nishio, Y.Morioka, N. Azuma, A. Ueno, H. Ohkita and M. Okada, Ind. Eng. Chem.Res., 34,4514.© 1995 ACS)

Songip et al.45 have studied the catalytic reforming of a heavy oil obtained bythermal degradation of PE at 450 0C. Prior to the catalytic treatment, the oil wasdistilled at 300 0C to remove the residue (20-30 wt%) and reduce its viscosity.Analysis of the oil showed it contained mostly paraffinic hydrocarbons. Figure5.19 shows the product distribution obtained in the catalytic conversion of thisoil in a fixed bed reactor at 400 0C over different acid catalysts. ZSM-5 zeolitewith Si/Al = 65 showed the highest activity, followed by zeolites REY and HY.Amorphous silica-alumina and a sample of ZSM-5 with very low Al content(Si/Al = 100) were the least active catalysts. Large differences were observedbetween the product distributions corresponding to the different zeolites. ZSM-5 with Si/Al = 65 gave the largest yield of gaseous products (69 wt%) and thelowest amount of gasoline, this fraction being even lower than in the raw oil.The gases were rich in C3 and C4 olefins. On the contrary, a high proportion ofgasoline fraction was produced over REY zeolite. Coke deposition was moresevere on HY and REY zeolites than over ZSM-5, which caused a fasterdeactivation of the former catalysts. The RON values corresponding to raw oiland the liquid products obtained after the catalytic treatments are shown inFigure 5.20. It is remarkable that the RON of the starting oil, produced by PEthermal degradation, is almost zero due to its high content of linear paraffins.As far as the gasoline fractions produced by catalytic reforming are concerned,the highest RON values correspond to the products formed over REY andZSM-5 zeolites, followed by the gasoline obtained over HY.

Yield

of s

tyren

e m

onom

er(w

t%)

Yield

of s

tyre

ne m

onom

er a

nd di

mer

(wt%

)

RON VALUEFigure 5.20 .ROiV values of the gasoline fraction obtained in the catalytic conversion of a

PEpyrolysis oil in a fixed bed reactor (T = 400 0C, WHSV = 1, t = 3 h).45

In further works, Songip et al.46A1 have studied the effect of the reactionconditions and the kinetics of the catalytic reforming of the PE oil over REYzeolites in a fixed bed reactor. Figures 5.21(A)-(Q show the yields of gasoline,gases and coke versus the oil conversion, obtained in experiments carried out atdifferent temperatures and space times. AU the data corresponding to the gasyield lie on the same curve, indicating that the temperature has no effect on thegas production, but that it depends mainly on the oil conversion. When morethan 80% of the starting oil has been converted, the gas yield grows exponen-

HY

REY

SILICA ALUMINA

ZSM-5 (1000)

ZSM-5 (65)

PEOIL

YIELD, WT %

Figure 5.19 Product distribution obtained in the catalytic conversion of a PE pyrolysis oilin a fixed bed reactor (T = 400 0C, WHSV = 1, t = 3 h).45

HY

REY

SILICA ALUMINA

ZSM-5 (1000)

ZSM-5 (65)

PEOIL

GASOLINE GAS COKE HEAVY OIL

Conversion of heavy oil [%]

Figure 5.21 Product distribution obtained in the catalytic conversion of a PE pyrolysis oilover REY zeolite:46 (A) gasoline yield, (B) gas yield, (C) coke yield.(Reprinted with permission from A.R. Songip, T. Masuda, H. Kuwaharaand K. Hashimoto, Energy Fuels, 8, 136. © 1994 ACS)

tially. At constant temperature, the gasoline yield exhibits a maximum oilconversion of about 80%, which suggests that the gasoline fraction is initiallyformed by cracking of the heavy oil and undergoes subsequent cracking to yieldgaseous hydrocarbons. Moreover, the gasoline yield also exhibits a maximumwith respect to temperature at about 400 0C. Coke deposition on the catalystincreases with the oil conversion, but is reduced by increasing the reactiontemperature.

The variation in gasoline quality with the temperature is shown in Figure5.22.46 Below 400 0C, the RON value increases with the temperature due to theincrease in the production of isoparaffins and the reduction in the content oflinear paraffins. However, at higher temperatures, cracking of the isoparaffinstakes place, leading to a certain reduction in the RON value.

Cok

e yi

eld

[wt%

lG

as y

ield

[%

]G

asol

ine

yiel

d [w

t%l

A)

B)

C)

Conversion of heavy oil [wt%]

Conversion of heavy oil [%]

573 K

623 K

673 K

723 K

573 K

623 K

673 K

723K

573 K

623 K

673 K

723 K

Temperature [K]

Figure 5.22 RON value and composition of the gasoline fraction produced in the catalyticconversion of a PE pyrolysis oil over REY zeolite, as functions of thetemperature.46

(Reprinted with permission from A.R. Songip, T. Masuda, H. Kuwaharaand K. Hashimoto, Energy Fuels, 8, 136. © 1994 ACS)

A number of two-step processes (thermal and catalytic treatments) have beenreported in the patent literature for the conversion of plastic wastes. Fukuda etai4S have proposed the thermal degradation of polyolefinic plastics in a stirredreactor at 420-470 0C, the volatile products being subsequently passed througha fixed bed reactor at 250-3400C, containing a ZSM-5 zeolite catalyst. Theprocess developed by Lechert et al.49 also consists of a two-reactor system:plastics are first pyrolysed above 600 0C in a sand fluidized bed reactor, and thegases produced are catalytically converted over ZSM-5 zeolite at 350-410 0C ina fixed bed reactor to increase the overall yield of liquid products.

An interesting process is that developed jointly by Fuji Recycle and Mobil Oilfor the treatment of plastic wastes containing PE, PP and PS (Figure 5.23).50'51

The waste plastics are crushed and washed to remove impurities (dirt, paper,etc.). Flotation in water is used to segregate the different polymers present in thewaste and to separate PVC and PET from PE, PP and PS. The resulting plasticmixture is warmed at 250 0C and introduced into the melting vessel at 300 0C bymeans of a heated extruder. The mixture is then transferred into the thermalcracking vessel where it is decomposed at about 400 0C. The gases generatedpass through the catalytic reactor, containing ZSM-5 zeolite, to be transformedinto higher value hydrocarbons. Final liquid and gas fractions are separated bycondensation, the gases being used as in-house fuel. A part of the product of thethermal cracking reactor is returned to the melting vessel, a settler beingavailable in the connecting pipeline to remove coke and other impurities.Processing PE and PP in this system typically yields 80% liquids, 15% gasesand 5% residue. The liquid is basically composed of 50% gasoline, 25%kerosene and 25% gas oil. When converting PS, the products are mainlyaromatic hydrocarbons, with a high content of ethylbenzene, toluene andbenzene.

Com

posi

tion

[w

t%],

RO

N [

-]

RON

ARIPi

NP2

Figure 5.23 Flow diagram of the Fuji Recycle-Mobil Oil process for the conversion ofplastic wastes by thermal-catalytic treatments. l

6 SummaryCatalytic cracking of plastic wastes offers a number of advantages comparedwith simple thermal degradation. The presence of catalysts greatly increases thepolymer cracking rate, which allows the use of lower temperatures and/orshorter reaction times. Moreover, by a suitable choice of catalysts it is possibleto direct the plastic degradation to the formation of different products: olefinicgases, gasoline fractions and middle distillates. Thus, the use of catalysts withstrong acidity in fluidized bed reactors has allowed high yields of gases rich in C3

and C4 olefins to be obtained, yields of up to 70% being reported attemperatures of about 400 0C. Likewise, the liquid hydrocarbons obtained inthe gasoline range are high quality products because they contain mainlybranched alkanes and aromatics, in contrast with the thermal degradation oils,which are formed by both linear alkanes and a-olefins.

However, these methods of plastic recycling are mainly limited to thedegradation of polyoleflnic plastics, because the presence in the feed of Cl orN-containing polymers may lead to a poisoning of the catalyst active sites.Likewise, the inorganic fillers and contaminants contained in the raw wastestend to remain with the solid catalysts, which means that further separationsteps are necessary.

The catalysts commonly used to promote plastic degradation are a variety ofacid solids such as amorphous silica-alumina, different types of zeolites,mesoporous aluminosilicates (MCM-41), sulfated zirconia, etc. Interestingresults have also been obtained in polymer cracking over activated carbons

CRACKED GAS

HEATTRANSFEROILFURNACE

SEHLER

CRACKINGFURNACE

CRACKEDOIL TOPRODUCTTANK

OIL/WATERSEPARATOR

EXTRUDER

HOPPER

MELTINGVESSEL

THERMALCRACKINGVESSEL

K.O.POT

CATALYTICREACTOR

GASRECYCLEFURNACE

CONDENSER

(alone or impregnated with transition metals), silica-based mesoporous materi-als (KFS-16), and basic solids. The activity and product distribution obtained inthe PE, PP and PS cracking over these materials depends mostly on the maincatalyst properties: type, strength and concentration of active sites, availablesurface area, average pore diameter, pore size distribution, etc. The bestcatalysts for the conversion of PE and PP are solid acids, although they usuallyundergo a significant deactivation due to coke deposition. However, in mostcases the initial activity can be restored by burning off the coke deposits. On theother hand, PS is degraded faster on basic catalysts, because acid materialspromote crosslinking reactions between the polymer chains.

The intense research effort carried out into the study of catalyst properties forthe conversion of plastic wastes is in contrast with the few studies that haveaddressed reactor design. Thus, most of the studies use batch or simple fixed bedreactors despite the heat transfer and flow problems associated with the lowthermal conductivity and high viscosity of the molten plastics. Various alter-natives have been proposed to solve these problems: the use of fluidized bedreactors, dissolution of the plastics in heavy oil fractions previously fed into thereactor, and a combination of thermal and catalytic treatments. However, allthese processes present a number of difficulties, which makes further work onthe reactor design necessary.

Likewise, it is convenient to check the behaviour of the catalysts whenworking with real plastic wastes rather than simply with model polymers, inorder to ascertain the effect of the various impurities and contaminants thewastes may contain on the catalyst activity and stability.

7 References1 H. Robson, Microporous Mater., 1998, 22(4-6), 495.2 R. Szostak, 'Molecular Sieves. Principles of Synthesis and Identification', Van

Nostrand Reinhold, New York, 1989.3 W.O. Haag, Stud. Surf. ScL Catal., 1994, 84, 1375.4 CT. Kresge, M.E. Leonowicz, WJ. Roth, J.C. Vartuli, and J.S. Beck, Nature, 1992,

359, 710.5 J.S. Beck, J.C. Vartuli, WJ. Roth, M.E. Leonowicz, CT. Kresge, K.D. Schmitt,

CT.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, and J.L.Schlenker, / . Am. Chem. Soc, 1992,114, 10834.

6 J. Aguado, D.P. Serrano, M.D. Romero, and J.M. Escola, Chem. Commun., 1996,1,725.

7 J. Aguado, J.L. Sotelo, D.P. Serrano, J.A. Calles, and J.M. Escola, Energy Fuels,1997, 11, 1225.

8 J. Kodaira, Z. Osawa, and H. Ando, J. Chem. Soc. Jpn, Chem. Ind. Chem., 1977,12,1892.

9 S.R. Ivanova, E.F. Gumerova, K.S. Minsker, G.E. Zaikov, and A.A. Berlin, Prog.Polym. ScL, 1990,15, 193.

10 P.L. Beltrame, P. Carniti, G. Audisio, and F. Bertini, Polym. Degrad. Stab., 1989,26,209.

11 R.C Mordi, R. Fields, and J. Dwyer, /. Anal. Appl. Pyrol, 1994, 29, 45.12 Y. Ishihara, H. Nambu, K. Saido, T. Ikemura, and T. Takesue, Polymer, 1992,

33(16), 3482.

13 Y. Ishihara, H. Nambu, T. Ikemura, and T. Takesue, J. Appl. Polym. ScL, 1989, 38,1491.

14 Y. Ishihara, H. Nambu, T. Ikemura, and T. Takesue, Fuel, 1990, 69, 978.15 M.A. Uddin, K. Koizumi, K. Murata, Y. Sakata, Polym. Degrad. Stab., 1997,56, 37.16 Y. Ishihara, H. Nambu, K. Saido, T. Ikemura, and T. Takesue, Bull. Chem. Soc. Jpn,

1991,64,3585.17 Y.-H. Lin, P.N. Sharratt, A.A. Garforth, and J. Dwyer, Thermochim. Ada, 1997,

294,45.18 Y. Uemichi, M. Hattori, T. Itoh, J. Nakamura, and M. Sugioka, Ind. Eng. Chem.

Res., 1998, 37, 867.19 R. Lin and R.L. White, J. Appl. Polym. ScL, 1995, 58, 1151.20 Y. Sakata, M.A. Uddin, A. Muto, Y. Kanada, K. Koizumi, and K. Murata, / . Anal.

Appl.Pyrol, 1997,43, 15.21 Y. Uemichi, Y. Makino, and T. Kanazuka, J. Anal. Appl. Pyrol, 1989,14, 331.22 D.S. Scott, S.R. Czernik, J. Piskorz, and D.St.A.G. Radlein, Energy Fuels, 1990, 4,

407.23 P.N. Sharratt, Y.-H. Lin, A.A. Garforth, and J. Dwyer, Ind. Eng. Chem. Res., 1997,

36,5118.24 S.H. Ng, Energy Fuels, 1995,9, 216.25 G. Audisio, F. Bertini, P.L. Beltrame, and P. Carniti, Makromol. Chem., Macromol.

Symp.,\992,Sl,\9\.26 Y. Sakata, M.A. Uddin, K. Koizumi, and K. Murata, Chem. Lett., 1996,1, 245.27 Y. Uemichi, Y. Kashiwaya, M. Tsukidate, A. Ayame, and H. Kanoh, Bull. Chem.

Soc. Jpn, 1983, 56, 2768.28 W. Zhao, S. Hasegawa, J. Fujita, F. Yoshii, T. Sasaki, K. Makkuuchi, J. Sun, and S.

Nishimoto, Polym. Degrad. Stab., 1996, 53, 129.29 D. P. Serrano, Proceedings of the 5th FEZA Euro workshop on 'Microporous

Materials: their Application in Catalysis', Gandia, Spain, 1998.30 J.L. Sotelo, R. van Grieken, J. Aguado, D.P. Serrano, J.M. Escola, and J.M.

Menendez, Proceedings of the 12th International Zeolite Conference, Baltimore,1998, ed. M.M.J. Treacey, B.K. Marcus, M.E. Bisher and J.B. Higgins, MaterialsResearch Society, 1999, Vol. II, p. 1441.

31 I. Nakamura and K. Fujimoto, Catal. Today, 1996, 27, 175.32 G. Audisio, F. Bertini, P.L. Beltrame, and P. Carniti, Polym. Degrad. Stab., 1990,29,

191.33 G. de Ia Puente, J.M. Arandes, and U.A. Sedran, Ind. Eng. Chem. Res., 1997, 36,

4530.34 D.P. Serrano, J. Aguado, and J.M. Escola (submitted to Appl. Catal. B: Environ.).35 J.W. Larsen, US Patent 3 996 022, 1976.36 R.C. Wingfield, Jr., J. Braslaw, and R.L. Gealer, US Patent 4458 095, 1984.37 R.C. Wingfield, Jr., J. Braslaw, and R.L. Gealer, US Patent 4 515 659, 1985.38 G.M. Platz, US Patent 5 504267, 1996.39 J.A. Butcher, Jr., US Patent 5 315055, 1994.40 N.Y. Chen and T.-Y. Yan, US Patent 4 108 730, 1978.41 K. Saito and M. Nanba, US Patent 4 584 421, 1986.42 RJ. Evans and H.L. Chum, US Patent 5 321 174, 1994.43 H. Ohkita, R. Nishiyama, Y. Tochihara, T. Mizushima, N. Kakuta, Y. Morioka, A.

Ueno, Y. Namiki, S. Tanifuji, H. Katoh, H. Sunazuka, R. Nakayama, and T.Kuroyanagi, Ind. Eng. Chem. Res., 1993, 32, 3112.

44 Z. Zhang, T. Hirose, S. Nishio, Y. Morioka, N. Azuma, A. Ueno, H. Ohkita, and M.Okada, Ind. Eng. Chem. Res., 1995, 34, 4514.

45 A.R. Songip, T. Masuda, H. Kuwahara, and K. Hashimoto, Appl. Catal. B: Environ.,1993,2,153.

46 A.R. Songip, T. Masuda, H. Kuwahara, and K. Hashimoto, Energy Fuels, 1994, 8,136.

47 A.R. Songip, T. Masuda, H. Kuwahara, and H. Hashimoto, Energy Fuels, 1994, 8,131.

48 T. Fukuda, K. Saito, S. Suzuki, H. Sato, and T. Hirota, US Patent 4851 601, 1989.49 H. Lechert, V. Woebs-Gosch, S. Qun, W. Kaminsky, and H. Sinn, US Patent

4871426,1989.50 T. Tachibana, Eur. Patent 675 189 Al, 1995.51 T. Hirota and F.N. Fagan, Makromol. Chem., Macromol Symp., 1992, 57, 161.