**3. Chemical recycling of polypropylene**

#### **3.1 Introduction**

Condensation polymers like PET or Nylon, can undergo chemolysis with different reagents to produce mainly the monomers from which they have been produced or other oligomers. In contrast, vinyl polymers, such as polyolefins (PP and PE) cannot be degraded with simple chemicals to their monomers due to the random schission of the C-C bonds. Two main chemical recycling routes are the thermal and catalytic degradation of these polymers. In thermal degradation, the process produces a broad product range and requires high operating temperatures, typically more than 500◦C and even up to 900 ◦C. Thermal cracking of polyethylene and polypropylene is usually carried out either in high temperatures (>700 ◦C), to produce an olefin mixture (C1–C4) and aromatic compounds (mainly benzene, toluene and xylene) or in low temperature (400–500 ◦C) (thermolysis) where three fractions are received: a high-calorific value gas, condensable hydrocarbon oil and waxes. In the first case, the objective is to maximize the gas fraction and to receive the olefins, which could be used after separation as monomers for the reproduction of the corresponding polyolefins. Cracking in lower temperatures leaves a waxy product in the reactor that mainly consists of paraffins together with a carbonized char. The gaseous fraction can be used for the supply of the energy required for the pyrolysis after burning. The liquid fraction mainly consists of linear olefins and paraffins with C11–C14 carbon atoms with only traces of aromatic compounds (Aguado and Serrano, 1999). Thermal cracking of polyolefins proceeds through a random scission mechanism in four steps: initiation, depropagation, inter- or intramolecular hydrogen transfer followed by β-scission and termination. In general, thermal cracking is more difficult in HDPE followed by LDPE and finally by PP.

Due to the low thermal conductivity of polymers together with the endotherm of cracking, thermal pyrolysis consumes large amounts of energy. Thus, catalytic technologies have been proposed to promote cracking at lower temperatures, resulting in reduced energy consumption and higher conversion rates. Furthermore, use of specific catalysts allows the process to be directed towards the formation of a narrower distribution of hydrocarbon products with a higher market value. Heterogeneous catalysis has been investigated extensively using solids with acid properties. Zeolites of the kind employed in the catalytic cracking of hydrocarbon feedstocks (Y, ZSM-5, Beta) as

Recently, recycling of PET using hydrolysis, glycolysis and aminolysis under microwave irradiation has been proposed (Achilias et al., 2010; Achilias et al., 2011; Siddiqui et al., 2010). PET recycling in a microwave reactor has been proved a very beneficial method

This section will not be presented in detail here because it is the subject of another chapter of this book. Interested reader can find extensive details on the techniques used for the chemical recycling of PET in several recent review papers appeared in literature (Scheirs,

Condensation polymers like PET or Nylon, can undergo chemolysis with different reagents to produce mainly the monomers from which they have been produced or other oligomers. In contrast, vinyl polymers, such as polyolefins (PP and PE) cannot be degraded with simple chemicals to their monomers due to the random schission of the C-C bonds. Two main chemical recycling routes are the thermal and catalytic degradation of these polymers. In thermal degradation, the process produces a broad product range and requires high operating temperatures, typically more than 500◦C and even up to 900 ◦C. Thermal cracking of polyethylene and polypropylene is usually carried out either in high temperatures (>700 ◦C), to produce an olefin mixture (C1–C4) and aromatic compounds (mainly benzene, toluene and xylene) or in low temperature (400–500 ◦C) (thermolysis) where three fractions are received: a high-calorific value gas, condensable hydrocarbon oil and waxes. In the first case, the objective is to maximize the gas fraction and to receive the olefins, which could be used after separation as monomers for the reproduction of the corresponding polyolefins. Cracking in lower temperatures leaves a waxy product in the reactor that mainly consists of paraffins together with a carbonized char. The gaseous fraction can be used for the supply of the energy required for the pyrolysis after burning. The liquid fraction mainly consists of linear olefins and paraffins with C11–C14 carbon atoms with only traces of aromatic compounds (Aguado and Serrano, 1999). Thermal cracking of polyolefins proceeds through a random scission mechanism in four steps: initiation, depropagation, inter- or intramolecular hydrogen transfer followed by β-scission and termination. In general, thermal

resulting not only in material recovery but also in substantial energy saving.

cracking is more difficult in HDPE followed by LDPE and finally by PP.

Due to the low thermal conductivity of polymers together with the endotherm of cracking, thermal pyrolysis consumes large amounts of energy. Thus, catalytic technologies have been proposed to promote cracking at lower temperatures, resulting in reduced energy consumption and higher conversion rates. Furthermore, use of specific catalysts allows the process to be directed towards the formation of a narrower distribution of hydrocarbon products with a higher market value. Heterogeneous catalysis has been investigated extensively using solids with acid properties. Zeolites of the kind employed in the catalytic cracking of hydrocarbon feedstocks (Y, ZSM-5, Beta) as

1998; Karayannidis and Achilias, 2007).

**3.1 Introduction** 

**3. Chemical recycling of polypropylene** 

well as other well-known acid solids like silica–alumina, alumina and clays are being the most studied. Mixtures of these catalysts like SAHA/ZSM-5, MCM-41/ZSM-5 have been also used. Cracking with acid catalysts takes place through the formation of carbocations, which requires the presence of strong acidic regions. Acid strength and textural properties are the main parameters dictating the performance of acid solids in the catalytic conversion of polymers. Porosity, surface area characteristics and particle size determine to a large extent the accessibility of bulky polymeric molecules to the internal catalytic acid sites of the solids. Thus, while catalyst HZSM-5 presents bigger reactivity from HMCM-41 in the cracking of HDPE and LDPE, at the decomposition of the large molecules of PP the transformation is almost the same with that of thermal cracking, because cross-section of polymer is very big in order to enter in catalysts' micropores (Achilias et al., 2007).

These facts strongly limit their applicability and especially increase the higher cost of feedstock recycling for waste plastic treatment. Therefore, catalytic degradation provides a means to address these problems. The addition of catalyst is expected to reduce decomposition temperature, to promote decomposition speed, and to modify the products. The catalytic degradation of polymeric materials has been reported for a range of model catalysts centred on the active components in a range of different model catalysts, including amorphous silica–aluminas, zeolites Y, mordinite and ZSM-5 and the family of mesoporous MCM-41 materials. However, these catalysts have been used that even if performing well, they can be unfeasible from the point of view of practical use due to the cost of manufacturing and the high sensitivity of the process to the cost of the catalyst. Another option for the chemical recycling of polymer wastes by using fluidized catalytic cracking (FCC) catalysts is attractive. Therefore, an alternative improvement of processing the recycling via catalytic cracking would operate in mixing the polymer waste with fluid catalytic cracking (FCC) commercial catalysts.

Recently, much attention has been paid to the recycling of waste polymers by thermal or catalytic pyrolysis as a method to recover value added products or energy via the production of high-value petrochemical feedstock or synthetic fuel fractions. The following review is rather selective and not extensive. Detailed reviews on the thermal and catalytic pyrolysis of PP based plastics can be found in an excellent recently published book by Scheirs and Kaminsky, 2006 and in Achilias et al., 2006.

#### **3.2 Pyrolysis**

Achilias et al., 2007*,* studied the technique of pyrolysis of polypropylene in a laboratory fixed bed reactor using as raw materials either model PP or waste products based on these polymer. The conclusions are very interesting. The oil and gaseous fractions recovered presented a mainly aliphatic composition consisting of a series of alkanes and alkenes of different carbon number with a great potential to be recycled back into the petrochemical industry as a feedstock for the production of new plastics or refined fuels. Details are presented in section 5.

Hayashi et al., 1998 studied pyrolysis of polypropylene in the presence of oxygen. The polypropylene was coated on porous α-alumina particles and then pyrolyzed in a flow of helium or a mixture of helium–oxygen at atmospheric pressure. The mass release from PP

Recent Advances in the Chemical Recycling

Brazilian crude oil (Assumpcao et al.**,** 2011).

temperature (oC)

Amount of PP (g) Pyrolysis

et al.**,** 2011).

Amount of PP (g)

Pyrolysis temperature (oC)

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 9

In the Diesel distillation range

400 0.0 39 n.d. 61 0.2 40 n.d. 60 0.4 52 n.d. 48 0.6 48 1.0 52 0.8 62 n.d. 38 1.0 59 n.d. 41 450 0.0 35 n.d. 65 0.2 30 1.0 69 0.4 32 n.d. 68 0.6 37 2.0 61 0.8 51 1.0 48 1.0 n.d. n.d. n.d. 500 0.0 31 n.d. 69 0.2 36 1.0 63 0.4 33 n.d. 67 0.6 34 6.0 60 0.8 40 2.0 58 1.0 n.d. n.d. n.d. Table 1. Yield of pyrolytic oil present at the liquid boiling point during PP co-pyrolysis with

> 0.0 400 59 30 11 450 65 26 9 500 88 10 2 0.2 400 68 28 4 450 86 11 3 500 95 4 1 0.4 400 79 13 4 450 83 10 7 500 90 3 7 0.6 400 83 14 3 450 91 5 4 500 97 2 1 0.8 400 17 81 2 450 23 74 3 500 50 49 1 1.0 400 15 84 1 450 17 82 1 500 25 73 3

Table 2. Yield of pyrolytic products in mixtures of PP with Brazilian crude oil*.* (Assumpcao

Yield of pyrolytic oil present at the liquid boiling point (%)

Below the Diesel distillation

Yield of pyrolytic products (%) Liquid Solid Gas

Above the Diesel distillation

was dramatically enhanced in the presence of oxygen at temperatures in the range of 200– 300oC. The net mass release rate in the presence of oxygen followed first-order kinetics with respect to the oxygen partial pressure and was controlled by the formation of peroxide on tertiary carbon of PP. The activation energy was 60–70 kJ/mol. The oxidative pyrolysis at 250oC converted 90% of PP into volatiles which mainly consisted of CS -soluble oils having a number-average chain length of 10.

Dawood et al. 2001 studied the influence of γ-irradiation on the thermal degradation of polypropylene by performing thermogravimetric analysis at three constant heating rates and at a constant temperature. At all the heating rates it can be indicated that the TG curves of the irradiated samples shifted to lower temperatures in comparison with the unirradiated one. The shift clearly increased with increasing irradiation dose, which means that the pyrolysis was enhanced by the irradiation. Since the difference in TG curves between the unirradiated sample and the samples irradiated to 10 and 30 kGy is quite large, small dose of irradiation is judged to be enough to cause a significant enhancement of the pyrolysis activity. The samples irradiated to small doses, 10 and 30 kGy, seem to show a pyrolysis behavior different from the other irradiated samples. At a small heating rate of 3 K/min, the TG curves of 10 and 30 kGy samples are close to the TG curve of 60 kGy sample, whereas the former TG curves are distinctly different from the latter TG curve at 10 K/min. These results may suggest that the mechanism of the increase in pyrolysis activity is different among the irradiated samples. A further examination of the influence of irradiation was performed by pyrolyzing the samples at a constant temperature. Similar to the case of dynamic heating rate, the difference in pyrolysis reactivity between the unirradiated and the 30 kGy irradiated sample is quite large, while the difference between the irradiated samples is small. This supports the suggestion that a small radiation dose is enough to cause a significant enhancement on the pyrolysis activity of PP.

#### **3.3 Co-pyrolysis**

Assumpcao et al., 2011, considered co-pyrolysis of PP with Brazilian crude oil by varying the temperature (400◦C to 500◦C) and the amount of PP fed to the reactor. The co-pyrolysis of plastic waste in an inert atmosphere provided around 80% of oil pyrolytic, and of these, half represent the fraction of diesel oil. this technique is a promise for PP waste recycling as it not only minimizes the environmental impact caused by inadequate disposal of this residues, but it also allows the reuse of a non-renewable natural resource (petroleum) through the use of diesel oil fractions obtained in this process. According to the results, the temperature increase has favored the increase of pyrolytic liquid generation and the reduction of the solid formed (Table 1). On the other hand, a huge increase in the PP amount has caused a decrease in total yield (liquid product) (Table 2). In general, it was observed that with temperature increase, there was a small reduction in yield in the diesel distillation range. Moreover, most part of these liquid distillates in a range higher than diesel, corresponding to heavy vacuum gas oil (GOP). This product (GOP) can still be cracked in an FCC generating more profitable products (naphtha and LPG), or can be used as fuel oil. The increase of PP in the reaction favors a yield increase in the diesel distillation range compared to pyrolysis of pure heavy oil, also forming a significant amount of compounds with distillation range lower than diesel.

was dramatically enhanced in the presence of oxygen at temperatures in the range of 200– 300oC. The net mass release rate in the presence of oxygen followed first-order kinetics with respect to the oxygen partial pressure and was controlled by the formation of peroxide on tertiary carbon of PP. The activation energy was 60–70 kJ/mol. The oxidative pyrolysis at 250oC converted 90% of PP into volatiles which mainly consisted of CS -soluble oils having a

Dawood et al. 2001 studied the influence of γ-irradiation on the thermal degradation of polypropylene by performing thermogravimetric analysis at three constant heating rates and at a constant temperature. At all the heating rates it can be indicated that the TG curves of the irradiated samples shifted to lower temperatures in comparison with the unirradiated one. The shift clearly increased with increasing irradiation dose, which means that the pyrolysis was enhanced by the irradiation. Since the difference in TG curves between the unirradiated sample and the samples irradiated to 10 and 30 kGy is quite large, small dose of irradiation is judged to be enough to cause a significant enhancement of the pyrolysis activity. The samples irradiated to small doses, 10 and 30 kGy, seem to show a pyrolysis behavior different from the other irradiated samples. At a small heating rate of 3 K/min, the TG curves of 10 and 30 kGy samples are close to the TG curve of 60 kGy sample, whereas the former TG curves are distinctly different from the latter TG curve at 10 K/min. These results may suggest that the mechanism of the increase in pyrolysis activity is different among the irradiated samples. A further examination of the influence of irradiation was performed by pyrolyzing the samples at a constant temperature. Similar to the case of dynamic heating rate, the difference in pyrolysis reactivity between the unirradiated and the 30 kGy irradiated sample is quite large, while the difference between the irradiated samples is small. This supports the suggestion that a small radiation dose is enough to cause a

Assumpcao et al., 2011, considered co-pyrolysis of PP with Brazilian crude oil by varying the temperature (400◦C to 500◦C) and the amount of PP fed to the reactor. The co-pyrolysis of plastic waste in an inert atmosphere provided around 80% of oil pyrolytic, and of these, half represent the fraction of diesel oil. this technique is a promise for PP waste recycling as it not only minimizes the environmental impact caused by inadequate disposal of this residues, but it also allows the reuse of a non-renewable natural resource (petroleum) through the use of diesel oil fractions obtained in this process. According to the results, the temperature increase has favored the increase of pyrolytic liquid generation and the reduction of the solid formed (Table 1). On the other hand, a huge increase in the PP amount has caused a decrease in total yield (liquid product) (Table 2). In general, it was observed that with temperature increase, there was a small reduction in yield in the diesel distillation range. Moreover, most part of these liquid distillates in a range higher than diesel, corresponding to heavy vacuum gas oil (GOP). This product (GOP) can still be cracked in an FCC generating more profitable products (naphtha and LPG), or can be used as fuel oil. The increase of PP in the reaction favors a yield increase in the diesel distillation range compared to pyrolysis of pure heavy oil, also forming a significant amount of compounds with

number-average chain length of 10.

significant enhancement on the pyrolysis activity of PP.

**3.3 Co-pyrolysis** 

distillation range lower than diesel.


Table 1. Yield of pyrolytic oil present at the liquid boiling point during PP co-pyrolysis with Brazilian crude oil (Assumpcao et al.**,** 2011).


Table 2. Yield of pyrolytic products in mixtures of PP with Brazilian crude oil*.* (Assumpcao et al.**,** 2011).

Recent Advances in the Chemical Recycling

**3.4 Catalyting cracking** 

found in the presence of the HY zeolite.

feedstocks on already existing industrial ethylene units.

of the studies reported in the literature will be reviewed below.

on, could possibly be obtained in addition to traditional hydrocarbons.

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 11

The decomposition of polyalkene oil/waxes during copyrolysis was confirmed. It was shown that the yields of the desired alkenes propene increased or slightly decreased compared to the yields from naphtha. In addition to the primary reactions, the secondary reactions leading to coke formation have also been studied. Slightly higher formation of coke was obtained at PP wax solution at the beginning of the measurements, on the clean surface of the reactor. After a thin layer of coke covered the walls, the production was the same as that from naphtha. The results confirm the possibility of polyalkenes recycling via the copyrolysis of polyalkene oils and waxes with conventional liquid steam cracking

A large number of laboratory studies have been conducted for the direct catalytic cracking of different type of plastics. A large variety of catalysts have been used that even if performing well, they can be unrealistic from the point of view of practical use due to the cost of manufacturing and the high sensitivity of the process to the cost of the catalyst. Some

Zhao et al., 1996 have studied the effects of different zeolites as H-Y, Na-Y, L, H-mordenite and Na-mordenite on the catalytic degradation of PP by thermogravimetry under nitrogen flow. It was found that the degradation temperature of PP strongly depended on the type of zeolite used and the amount added. One type of HY zeolite (320HOA) was shown to be a very effective catalyst. Pyrolysis products, which were identified by using a coupled gaschromatograph-mass-spectrometer, were also affected by the addition of zeolites. Some zeolites did not change the structure of the products but narrowed the product distribution to a smaller molecule region, while the HYzeolite led to hydrocarbons concentrated at those containing 4-9 carbons. Furthermore, some new compounds with cyclic structures were

Also, Zhao studied the effect of irradiation on pyrolysis of polypropylene in the presence of zeolites the results revealed that thermal degradation temperature of PP was significantly reduced when PP was irradiated in the presence of a zeolite. The irradiation-induced temperature reduction depended on the zeolite structure and composition, as well as on the morphology of the mixture. Identification of pyrolysis products indicated that, in the absence of zeolite, irradiation resulted only in a change of the product distribution but no formation of new compounds. In the presence of zeolite, however, a series of oxidized products were formed. In addition, the pyrolysis could be performed at a much lower temperature**.** Irradiation is able to render PP much moresusceptible to thermal degradation when carried out in the presence of zeolite. However, this effect was closely related to the type of zeolites, mixing methods and irradiation conditions.Furthermore, in pyrolysis of properly irradiated PP-zeolite mixtures, new chemicals such as acetone, acetic acid and so

Ishihara et al., 1993 investigated the catalytic degradation of PP by silica–alumina at temperatures between 180 and 300°C in a semibatch reactor under a nitrogen flow. The production of gas precursors was found essential to decomposition. The most important elementary reaction is the intramolecular rearrangement of chain-end secondary carbonium ions in the liquid fraction to inner tertiary carbon atoms. The catalytic decomposition of

Ballice et al**.,** 2002 investigated the temperature-programmed co-pyrolysis of Soma-lignites form Turkey with PP. A series co-pyrolysis operation was performed with lignites and PP using a 1:3, 1:1, 3:1 total carbon ratio of lignites to plastic. A fixed bed reactor was used to pyrolyse small sample of lignites and PP mixture under an inert gas flow (argon). In addition, the performance of the experimental apparatus was investigated by establishing a carbon balance and the degree of recovery of total organic carbon of the samples as aliphatic hydrocarbons and in solid residue was determined. Conversion into volatile hydrocarbons was found higher with increasing PP ratio in lignites–PP system while C16+ hydrocarbons and the amount of coke deposit were lower in the presence of PP. The maximum product release temperature was found to be approximately 440 °C for co-pyrolysis of lignite–PP. Straight- and branched-chain paraffins and olefins from methane to C26, diene and simple aromatic hydrocarbons were determined in co-pyrolysis products. The fraction of *n*paraffins was higher than that of 1-olefins at a high proportion of lignite in the mixture.

Co-pyrolysis of lignite with PP has been found to give less C16+ *n*-paraffins and 1-olefins than pyrolysis of lignite by increasing PP ratios. Coke deposit in co-processing decreased also by increasing PP ratios. The *n*-paraffins were found to consist of mainly C1–C9, and relatively small amount of C10–C15 and C16+ fractions. The evolution of 1-olefins decreased in co-pyrolysis operation because of the higher hydrogen content in feed by increasing ratios of PP. A slightly synergistic effect were determined in the co-pyrolysis operation and the experimental results indicated that the pyrolysis products of PP are in highly aliphatic character, and during the initial stages of pyrolysis, these pyrolysis products of PP is expected to be a relative poor solvent for the structures of lignite. In addition, relative to liquefaction sources materials such as coals, the dominant components of municipal solid wastes (mainly PE, PS, PET and PP) are hydrogen rich so that co-processing of coal with waste plastics could be a good way to recycle waste plastics into useful products (Table 3).


Table 3. 1-Olefins distribution in co-pyrolysis of lignite with PP (Ballice et al.**,** 2002).

Hajekova and Bajus, 2005 investigated the thermal decomposition of polyalkenes as a recycling route for the production of petrochemical feedstock. Polypropylene was thermally decomposed individually in a batch reactor at 450o C, thus forming oil/wax products. Then the product was dissolved in primary heavy naphtha to obtain steam cracking feedstock. The selectivity and kinetics of copyrolysis for 10 mass% solutions of oil/waxes from PP with naphtha in the temperature range from 740 to 820 oC at residence times from 0.09 to 0.54 s were studied.

The decomposition of polyalkene oil/waxes during copyrolysis was confirmed. It was shown that the yields of the desired alkenes propene increased or slightly decreased compared to the yields from naphtha. In addition to the primary reactions, the secondary reactions leading to coke formation have also been studied. Slightly higher formation of coke was obtained at PP wax solution at the beginning of the measurements, on the clean surface of the reactor. After a thin layer of coke covered the walls, the production was the same as that from naphtha. The results confirm the possibility of polyalkenes recycling via the copyrolysis of polyalkene oils and waxes with conventional liquid steam cracking feedstocks on already existing industrial ethylene units.

#### **3.4 Catalyting cracking**

10 Material Recycling – Trends and Perspectives

Ballice et al**.,** 2002 investigated the temperature-programmed co-pyrolysis of Soma-lignites form Turkey with PP. A series co-pyrolysis operation was performed with lignites and PP using a 1:3, 1:1, 3:1 total carbon ratio of lignites to plastic. A fixed bed reactor was used to pyrolyse small sample of lignites and PP mixture under an inert gas flow (argon). In addition, the performance of the experimental apparatus was investigated by establishing a carbon balance and the degree of recovery of total organic carbon of the samples as aliphatic hydrocarbons and in solid residue was determined. Conversion into volatile hydrocarbons was found higher with increasing PP ratio in lignites–PP system while C16+ hydrocarbons and the amount of coke deposit were lower in the presence of PP. The maximum product release temperature was found to be approximately 440 °C for co-pyrolysis of lignite–PP. Straight- and branched-chain paraffins and olefins from methane to C26, diene and simple aromatic hydrocarbons were determined in co-pyrolysis products. The fraction of *n*paraffins was higher than that of 1-olefins at a high proportion of lignite in the mixture.

Co-pyrolysis of lignite with PP has been found to give less C16+ *n*-paraffins and 1-olefins than pyrolysis of lignite by increasing PP ratios. Coke deposit in co-processing decreased also by increasing PP ratios. The *n*-paraffins were found to consist of mainly C1–C9, and relatively small amount of C10–C15 and C16+ fractions. The evolution of 1-olefins decreased in co-pyrolysis operation because of the higher hydrogen content in feed by increasing ratios of PP. A slightly synergistic effect were determined in the co-pyrolysis operation and the experimental results indicated that the pyrolysis products of PP are in highly aliphatic character, and during the initial stages of pyrolysis, these pyrolysis products of PP is expected to be a relative poor solvent for the structures of lignite. In addition, relative to liquefaction sources materials such as coals, the dominant components of municipal solid wastes (mainly PE, PS, PET and PP) are hydrogen rich so that co-processing of coal with waste plastics could be a good way to recycle waste

Lignite PP Lignite-PP

C2 – C4 57.8 81.0 58.3 58.2 59.7 C5-C9 15.6 19.0 32.6 30.1 26.8 C10-C15 14.7 - 4.2 4.0 3.7 C16+ 11.9 - 4.9 8.0 9.8

1-Olefins 10.2 10.0 14.4 13.8 13.4

Hajekova and Bajus, 2005 investigated the thermal decomposition of polyalkenes as a recycling route for the production of petrochemical feedstock. Polypropylene was thermally decomposed individually in a batch reactor at 450o C, thus forming oil/wax products. Then the product was dissolved in primary heavy naphtha to obtain steam cracking feedstock. The selectivity and kinetics of copyrolysis for 10 mass% solutions of oil/waxes from PP with naphtha in the temperature range from 740 to 820 oC at residence times from 0.09 to 0.54 s

Table 3. 1-Olefins distribution in co-pyrolysis of lignite with PP (Ballice et al.**,** 2002).

(1:3)

Lignite-PP (1:1)

Lignite-PP (3:1)

plastics into useful products (Table 3).

wt.% relative to all the aliphatic hydrocarbons

wt.% relative to the 1-olefins

Hydrocarbon fraction

were studied.

A large number of laboratory studies have been conducted for the direct catalytic cracking of different type of plastics. A large variety of catalysts have been used that even if performing well, they can be unrealistic from the point of view of practical use due to the cost of manufacturing and the high sensitivity of the process to the cost of the catalyst. Some of the studies reported in the literature will be reviewed below.

Zhao et al., 1996 have studied the effects of different zeolites as H-Y, Na-Y, L, H-mordenite and Na-mordenite on the catalytic degradation of PP by thermogravimetry under nitrogen flow. It was found that the degradation temperature of PP strongly depended on the type of zeolite used and the amount added. One type of HY zeolite (320HOA) was shown to be a very effective catalyst. Pyrolysis products, which were identified by using a coupled gaschromatograph-mass-spectrometer, were also affected by the addition of zeolites. Some zeolites did not change the structure of the products but narrowed the product distribution to a smaller molecule region, while the HYzeolite led to hydrocarbons concentrated at those containing 4-9 carbons. Furthermore, some new compounds with cyclic structures were found in the presence of the HY zeolite.

Also, Zhao studied the effect of irradiation on pyrolysis of polypropylene in the presence of zeolites the results revealed that thermal degradation temperature of PP was significantly reduced when PP was irradiated in the presence of a zeolite. The irradiation-induced temperature reduction depended on the zeolite structure and composition, as well as on the morphology of the mixture. Identification of pyrolysis products indicated that, in the absence of zeolite, irradiation resulted only in a change of the product distribution but no formation of new compounds. In the presence of zeolite, however, a series of oxidized products were formed. In addition, the pyrolysis could be performed at a much lower temperature**.** Irradiation is able to render PP much moresusceptible to thermal degradation when carried out in the presence of zeolite. However, this effect was closely related to the type of zeolites, mixing methods and irradiation conditions.Furthermore, in pyrolysis of properly irradiated PP-zeolite mixtures, new chemicals such as acetone, acetic acid and so on, could possibly be obtained in addition to traditional hydrocarbons.

Ishihara et al., 1993 investigated the catalytic degradation of PP by silica–alumina at temperatures between 180 and 300°C in a semibatch reactor under a nitrogen flow. The production of gas precursors was found essential to decomposition. The most important elementary reaction is the intramolecular rearrangement of chain-end secondary carbonium ions in the liquid fraction to inner tertiary carbon atoms. The catalytic decomposition of

Recent Advances in the Chemical Recycling

Distribution of gaseous products (wt % feed)

all catalysts studied (Table 4).

Yield (wt.% feed)

Hydrocarbon gases

(C1-C4)

contact times.

C7 range.

and 6).

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 13

Greater product selectivity was observed with HZSM-5 and HMOR as catalysts with about 60% of the product in the C3-C5 range and HMOR generating the highest yield of i-C4 for

Gas 89.49 94.77 88.29 86.44 86.19 Liquid 3.75 2.31 4.54 4.73 5.07 Residue 6.76 3.92 7.17 9.83 8.74 Involatile residue 3.32 2.27 4.96 7.49 6.83 Coke 3.44 1.25 3.21 2.34 1.91 Mass balance (%) 93.71 95.32 93.24 89.68 88.46

Gasoline (C5-C9) 51.83 25.54 27.95 63.65 60.56 BTX 0.93 1.82 0.48 0.25 0.16

The larger pore zeolites (HUSY and HMOR) showed deactivation in contrast to the more restrictive HZSM-5. Observed differences in product yields and product distributions under identical reaction conditions can be attributed to the microstructure of catalysts.Valuable hydrocarbons of olefins and iso-olefins were produced by low temperatures and short

Lin and Yang, 2007 pyrolysed PP over spent FCC commercial catalyst (FCC-s1) using a laboratory fluidised-bed reactor operating isothermally at ambient pressure. The yield of gaseous and liquid hydrocarbon products at 390oC for spent FCC commercial catalyst (87.8 wt%) gave much higher yield than silicate (only 17.1 wt%). Greater product selectivity was observed with FCC-s1 as a post-use catalyst with about 61 wt% olefins products in the C3 -

The use of fluidised-bed reaction system coped with a spent FCC equilibrium catalyst can be a better option from economical point of view since it can gives a good conversion with comparable short reaction time, and even its activity is lower than that of the zeolites (ZSM-5 and HUSY) and silica-aluminas (SAHA), this can be compensated by increasing the catalyst to PP ratio. Product distributions with FCC-s1 catalyst contained more olefinic materials in the range of C3-C7 (about 56 wt% at 390 oC). It was concluded that the use of spent FCC commercial catalyst and under appropriate reaction conditions can have the ability to control both the product yield and product distribution from polymer degradation, potentially leading to a cheaper process with more valuable products (Tables 5

Table 4. PP degradation products depending on catalysts type (Lin and Yen, 2005).

HUSY HZSM-5 HMOR SAHA MCM-41

36.73 67.41 59.86 22.54 25.47

Degradation results Catalyst type

polypropylene proceeds as follows: polymer -degraded polymer +oligomer+ liquid+gas. Gas is produced from the chain-ends of the liquid fraction and its components are primarily isobutene and isopentane. The most important elementary reaction in the decomposition is intramolecular rearrangement taking place via a six-membered transition state to inner tertiary carbon atoms (back-biting reactions). The main gas components are produced by the decomposition of the C, fraction formed by the back-biting reaction.

Durmus et al., 2005 studied thermal-catalytic degradation kinetics of polypropylene over BEA, ZSM-5 and MOR zeolites. Degradation rate of the PP over zeolites was studied by thermogravimetric analysis (TGA) employing four different heating rates and apparent activation energies of the processes were determined by the Kissinger equation. The catalytic activity of zeolites decreases as BEA > ZSM-5a (Si/Al = 12.5) > ZSM-5b (Si/Al = 25) > MOR depending on pore size and acidity of the catalysts. On the other hand, initial degradation is relatively faster over MOR and BEA than that over both ZSM-5 catalysts depending on the apparent activation energy. It can be concluded that acidity of the catalyst is the most important parameter in determining the activity for polymer degradation process as well as other structural parameters, such as pore structure and size.

Lin et al., 2005 have investigated the catalytic cracking of PP in a fluidized bed reactor using H-ZSM-5, H-USY, H-mordenite, silica–alumina and MCM-41, with nitrogen as fluidizing gas (Figure 1). PP was pyrolysed over various catalysts using a laboratory fluidised-bed reactor operating isothermally at ambient pressure. The yield of volatile hydrocarbons for zeolite catalysts was higher than that for non-zeolite catalysts. Product distributions with HZSM-5 contained more olefinic materials with about 60 wt% in the range of C3 - C5. However, both HMOR and HUSY produced more paraffin streams with large amounts of isobutene (i-C4) and both catalysts were deactivated during the course of the degradation. SAHA and MCM-41 showed the lowest conversion and generated an olefin-rich product with a rise to the broadest carbon range of C3 - C7.

Fig. 1. Schematic diagram of catalytic fluidized-bed reactor system (Lin and Yen 2005).

polypropylene proceeds as follows: polymer -degraded polymer +oligomer+ liquid+gas. Gas is produced from the chain-ends of the liquid fraction and its components are primarily isobutene and isopentane. The most important elementary reaction in the decomposition is intramolecular rearrangement taking place via a six-membered transition state to inner tertiary carbon atoms (back-biting reactions). The main gas components are produced by the

Durmus et al., 2005 studied thermal-catalytic degradation kinetics of polypropylene over BEA, ZSM-5 and MOR zeolites. Degradation rate of the PP over zeolites was studied by thermogravimetric analysis (TGA) employing four different heating rates and apparent activation energies of the processes were determined by the Kissinger equation. The catalytic activity of zeolites decreases as BEA > ZSM-5a (Si/Al = 12.5) > ZSM-5b (Si/Al = 25) > MOR depending on pore size and acidity of the catalysts. On the other hand, initial degradation is relatively faster over MOR and BEA than that over both ZSM-5 catalysts depending on the apparent activation energy. It can be concluded that acidity of the catalyst is the most important parameter in determining the activity for polymer degradation

Lin et al., 2005 have investigated the catalytic cracking of PP in a fluidized bed reactor using H-ZSM-5, H-USY, H-mordenite, silica–alumina and MCM-41, with nitrogen as fluidizing gas (Figure 1). PP was pyrolysed over various catalysts using a laboratory fluidised-bed reactor operating isothermally at ambient pressure. The yield of volatile hydrocarbons for zeolite catalysts was higher than that for non-zeolite catalysts. Product distributions with HZSM-5 contained more olefinic materials with about 60 wt% in the range of C3 - C5. However, both HMOR and HUSY produced more paraffin streams with large amounts of isobutene (i-C4) and both catalysts were deactivated during the course of the degradation. SAHA and MCM-41 showed the lowest conversion and generated an olefin-rich product

Fig. 1. Schematic diagram of catalytic fluidized-bed reactor system (Lin and Yen 2005).

decomposition of the C, fraction formed by the back-biting reaction.

process as well as other structural parameters, such as pore structure and size.

with a rise to the broadest carbon range of C3 - C7.


Greater product selectivity was observed with HZSM-5 and HMOR as catalysts with about 60% of the product in the C3-C5 range and HMOR generating the highest yield of i-C4 for all catalysts studied (Table 4).

Table 4. PP degradation products depending on catalysts type (Lin and Yen, 2005).

The larger pore zeolites (HUSY and HMOR) showed deactivation in contrast to the more restrictive HZSM-5. Observed differences in product yields and product distributions under identical reaction conditions can be attributed to the microstructure of catalysts.Valuable hydrocarbons of olefins and iso-olefins were produced by low temperatures and short contact times.

Lin and Yang, 2007 pyrolysed PP over spent FCC commercial catalyst (FCC-s1) using a laboratory fluidised-bed reactor operating isothermally at ambient pressure. The yield of gaseous and liquid hydrocarbon products at 390oC for spent FCC commercial catalyst (87.8 wt%) gave much higher yield than silicate (only 17.1 wt%). Greater product selectivity was observed with FCC-s1 as a post-use catalyst with about 61 wt% olefins products in the C3 - C7 range.

The use of fluidised-bed reaction system coped with a spent FCC equilibrium catalyst can be a better option from economical point of view since it can gives a good conversion with comparable short reaction time, and even its activity is lower than that of the zeolites (ZSM-5 and HUSY) and silica-aluminas (SAHA), this can be compensated by increasing the catalyst to PP ratio. Product distributions with FCC-s1 catalyst contained more olefinic materials in the range of C3-C7 (about 56 wt% at 390 oC). It was concluded that the use of spent FCC commercial catalyst and under appropriate reaction conditions can have the ability to control both the product yield and product distribution from polymer degradation, potentially leading to a cheaper process with more valuable products (Tables 5 and 6).

Recent Advances in the Chemical Recycling

Yield (wt.%)

Al-MCM-48 (Si/Al=60)

Al-MCM-48 (Si/Al=60)

and PP:catalyst=5:1 (Park et al., 2008).

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 15

than that from polyethylene. Pt metal was more effective than Fe only when the reaction conditions involved a longer contact time. The formation of aromatics was explained by essentially the same mechanism as the case of polyethylene, in which an influence of methyl branching of polyropylene on the aromatization yield and a difference in catalytic activity of

Park et al., 2008 reported pyrolysis of polypropylene over mesoporous MCM-48 material. Mesoporous MCM-48 materials were employed as catalysts for the degradation of PP. The catalytic activity of Al-MCM-48 was much higher than that of Si-MCM-48. Al-MCM-48 mainly generated C7–C10 hydrocarbons, while Si-MCM-48 exhibited a relatively broader distribution of oil products (C7–C14). Al-MCM-48 showed high catalytic stability for the degradation of PP. In view of these facts, Al-MCM-48 can be considered a promising

Conversion (%) 3.3 75.3 90.2 95.7

Oil 2.1 58.3 72.2 76.5 Gas 1.2 17.0 18.0 19.2 Table 7. Effect of catalyst type on product yield obtained from PP pyrolysis at 380oC for 1 h

Without catalyst 0.9 3.6 1.0 2.3 20.4 71.8 Si-MCM-48 0.1 0.1 0.1 0.4 14.0 85.3

Table 8. Effect of catalyst type on the product distribution of the gas fraction during PP

Cardona and Corma, 2000 studied the tertiary recycling of polypropylene by catalytic cracking in a semibatch stirred reactor semicontinuous reactor has been presented that allows carrying out efficiently the catalytic cracking of PP. By working with USY zeolites with different unit cell sizes, it has been proven that neither the total amount nor the strength of the acid sites are the most determinant factors for cracking PP. the first cracking event of PP occurs at or close to the external surface. Then the formation of mesopores in the zeolite strongly improves the cracking activity. This has been supported by the results obtained with a Y zeolite synthesized with smaller crystallite sizes (Table 9). Finally it has been shown that amorphous or ordered silica–aluminas are very active catalysts. However, a FCC equilibrium catalyst can be a better option from an economical point of view since it

Catalyst type

(Si/Al=60)

Al-MCM-48 (Si/Al=30)

Si-MCM-48 Al-MCM-48

CH4 C2H6 C2H4 C3H8 C3H6 C4s

0.1 0.1 0.2 0.4 12.3 86.9

0.1 0.1 0.2 0.5 13.4 85.7

the catalysts containing Pt and Fe for a ring expansion reaction were considered.

catalyst for the degradation of other waste plastics (Tables 7 and 8).

Catalyst Product distribution (wt.%)

pyrolysis at 380oC for 1 h and PP:catalyst=5:1 (Park et al., 2008).

Without catalyst


Table 5. Products of PP pyrolysis with FCC catalyst (Lin and Yang, 2007).


Table 6. Effect of fluidizing N2 rates on the product yield in PP pyrolysis (Lin and Yang, 2007).

Dawood and Miura, 2002 have studied the effect of exposing PP to γ-irradiation prior to the catalytic pyrolysis over a HY-zeolite using a thermobalance and a semi-batch reactor. A significant increase in the rate of the catalytic pyrolysis was realized when PP was exposed to a small irradiation dose of 10 kGy. The high reactivity of the irradiated PP was conjugated with low yields of residue and coke in addition to enhanced selectivity for light distillate (C7–C10). Examining the effect of pyrolysis temperature revealed that the catalytic pyrolysis preferred high temperature among the investigated temperature range of 325–375 oC. The results presented above clarified that a significant increase in the rate of the catalytic pyrolysis with enhanced selectivity of C7–C10 compounds can be obtained by exposing PP to the ionizing irradiation prior to the catalytic pyrolysis. The results suggested the applicability of the proposed pyrolysis method for enhancing the catalytic conversion of plastic waste into useful hydrocarbons.

Uemichi et al, 1989 investigated the degradation of polypropylene to aromatic hydrocarbons over activated carbon catalysts containing Pt and Fe. The results obtained were compared with those for the degradation of polyethylene. The addition of Pt or Fe to activated carbon resulted in an increase in the yield of aromatics from polypropylene. However, the increase was less

Gas 26.7 27.8 26.9 27.6 28.2 Liquid 63.2 61.7 60.9 60.3 59.2 Gasoline (C5-C9) 57.4 55.4 54.3 52.9 51.4 Condensate liquid 4.9 5.3 5.4 5.8 6.0 BTX 0.9 1.0 1.2 1.6 1.8 Residue 10.1 10.5 12.2 12.1 12.6 Involatile residue 7.6 8.3 9.9 10.4 10.7 Coke 2.5 2.2 2.3 1.7 1.9 Mass balance (%) 89.5 90.6 91.8 92.5 90.4

Gas 29.6 28.8 26.9 26.3 26.1 Liquid 60.5 60.4 60.9 60.7 60.1 Gasoline (C5-C9) 55.1 54.7 54.3 54.6 53.7 Condensate liquid 4.9 4.9 5.4 4.5 4.3 BTX 0.5 0.8 1.2 1.6 2.1 Residue 9.9 10.8 12.2 13.0 13.8 Involatile residue 7.7 8.5 9.9 10.5 11.2 Coke 2.2 2.3 2.3 2.5 2.6 Mass balance (%) 89.2 89.6 91.8 90.3 94.1 Table 6. Effect of fluidizing N2 rates on the product yield in PP pyrolysis (Lin and Yang, 2007).

Dawood and Miura, 2002 have studied the effect of exposing PP to γ-irradiation prior to the catalytic pyrolysis over a HY-zeolite using a thermobalance and a semi-batch reactor. A significant increase in the rate of the catalytic pyrolysis was realized when PP was exposed to a small irradiation dose of 10 kGy. The high reactivity of the irradiated PP was conjugated with low yields of residue and coke in addition to enhanced selectivity for light distillate (C7–C10). Examining the effect of pyrolysis temperature revealed that the catalytic pyrolysis preferred high temperature among the investigated temperature range of 325–375 oC. The results presented above clarified that a significant increase in the rate of the catalytic pyrolysis with enhanced selectivity of C7–C10 compounds can be obtained by exposing PP to the ionizing irradiation prior to the catalytic pyrolysis. The results suggested the applicability of the proposed pyrolysis method for enhancing the catalytic conversion of

Uemichi et al, 1989 investigated the degradation of polypropylene to aromatic hydrocarbons over activated carbon catalysts containing Pt and Fe. The results obtained were compared with those for the degradation of polyethylene. The addition of Pt or Fe to activated carbon resulted in an increase in the yield of aromatics from polypropylene. However, the increase was less

10 20 30 40 60

900 750 600 450 300

Degradation results Ratio of polymer to catalyst (wt.%)

Table 5. Products of PP pyrolysis with FCC catalyst (Lin and Yang, 2007).

Degradation results Fluidizing N2 rates (mL/min)

Yield (wt.% feed)

Yield (wt.% feed)

plastic waste into useful hydrocarbons.

than that from polyethylene. Pt metal was more effective than Fe only when the reaction conditions involved a longer contact time. The formation of aromatics was explained by essentially the same mechanism as the case of polyethylene, in which an influence of methyl branching of polyropylene on the aromatization yield and a difference in catalytic activity of the catalysts containing Pt and Fe for a ring expansion reaction were considered.

Park et al., 2008 reported pyrolysis of polypropylene over mesoporous MCM-48 material. Mesoporous MCM-48 materials were employed as catalysts for the degradation of PP. The catalytic activity of Al-MCM-48 was much higher than that of Si-MCM-48. Al-MCM-48 mainly generated C7–C10 hydrocarbons, while Si-MCM-48 exhibited a relatively broader distribution of oil products (C7–C14). Al-MCM-48 showed high catalytic stability for the degradation of PP. In view of these facts, Al-MCM-48 can be considered a promising catalyst for the degradation of other waste plastics (Tables 7 and 8).




Table 8. Effect of catalyst type on the product distribution of the gas fraction during PP pyrolysis at 380oC for 1 h and PP:catalyst=5:1 (Park et al., 2008).

Cardona and Corma, 2000 studied the tertiary recycling of polypropylene by catalytic cracking in a semibatch stirred reactor semicontinuous reactor has been presented that allows carrying out efficiently the catalytic cracking of PP. By working with USY zeolites with different unit cell sizes, it has been proven that neither the total amount nor the strength of the acid sites are the most determinant factors for cracking PP. the first cracking event of PP occurs at or close to the external surface. Then the formation of mesopores in the zeolite strongly improves the cracking activity. This has been supported by the results obtained with a Y zeolite synthesized with smaller crystallite sizes (Table 9). Finally it has been shown that amorphous or ordered silica–aluminas are very active catalysts. However, a FCC equilibrium catalyst can be a better option from an economical point of view since it

Recent Advances in the Chemical Recycling

presented (Table 11).

None

**4. Chemical recycling of polystyrene** 

less than 0.5% of the solid waste going to landfills.

**4.1 Introduction** 

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 17

Panda et al., 2011 investigated catalytic performances of kaoline and silica alumina in the thermal degradation of polypropylene. Polypropylene was cracked thermally and catalytically in the presence of kaoline and silica alumina in a semi batch reactor in the temperature range 400–550ºC in order to obtain suitable liquid fuels. It was observed that up to 450ºC thermal cracking temperature, the major product of pyrolysis was liquid oil and the major product at other higher temperatures (475–550ºC) are viscous liquid or wax and the highest yield of pyrolysis product is 82.85% by weight at 500ºC. Use of kaoline and silica alumina decreased the reaction time and increased the yield of liquid fraction. Again the major pyrolysis product in catalytic pyrolysis at all temperatures was low viscous liquid oil. Silica alumina was found better as compared to kaoline in liquid yield and in reducing the reaction temperature. The maximum oil yield using silica alumina and kaoline catalyst are 91% and 89.5% respectively. On the basis of the obtained results hypothetical continuous process of waste polypropylene plastics processing for engine fuel production can be

In conclusion, catalytic pyrolysis of PP reduces environmental impacts, also the time of recycling and results in very useful products, with potential use as fuel replacements.

(thermal cracking)

Liquid product (wt.%) 82.85 89.50 91.0 Gaseous product (wt.%) 16.25 9.75 8.0 Solid residue (wt.%) 0.90 0.75 1.0 Density of oil at 15 oC (g/mL) 0.84 0.745 0.770 Viscosity of oil at 30 oC (cSt) 4.31 2.18 2.21

Polystyrene (PS) is widely used in the manufacture of many products due to its favorable properties such as good strength, light weight, and durability and is the material of choice for packaging various electronics and other fragile items. In general, PS accounts for about 9-10% of the plastic waste in municipal solid waste (MSW). In the past several years, PS has received much public and media attention. Polystyrene has been described by various environmental groups as being nondegradable, nonrecyclable, toxic when burned, landfillchoking, ozone-depleting, wildlife-killing, and even carcinogenic. These misconceptions regarding PS have resulted in boycotts and bans in various localities. Actually, PS comprises

Table 11. Pyrolysis of PP in optimum conditions (Panda and Singh, 2011).

Type of catalyst

Kaoline (PP:cat=3:1)

T = 500oC T = 450oC T = 500 oC

Silica-alumina (PP:cat=3:1)


gives a very good selectivity, and even if its activity is lower than that of the silica–aluminas, this can be compensated by increasing the catalyst to PP ratio.

Table 9. Effect of the catalyst type on the product distribution during PP pyrolysis at 380 oC (Cardona and Corma, 2000).

Xie et al., 2008 have reported catalytic cracking of polypropylene (PP) over MCM-41 modified by Zr and Mo. The relationship among structure, acidity and catalytic activity of Zr–Mo-MCM-41 was studied. The results showed that Zr–Mo-MCM-41 exhibited high activity for the cracking of PP and good selectivity for producing liquid hydrocarbons of higher carbon numbers. The results were compared with those obtained over HZSM-5, SiO2–Al2O3 and other MCM-41 mesoporous molecular sieves. For the catalytic cracking of PP, Mo enhances the selectivity to high carbon number hydrocarbons and Zr enhances the acidity of catalyst and results in the increasing cracking conversion of PP. Zr–Mo-MCM-41 using Zr(SO4)2 as Zr source is of the best catalytic activity and selectivity to high carbon number hydrocarbon, which means that Zr–Mo-MCM-41 will probably become good potential catalysts for the cracking of PP (Table 10).


Table 10. Catalytic activities of different catalyst in PP pyrolysis for 30 min at catalyst/ PP = 0.01 (Xie et al., 2008).

gives a very good selectivity, and even if its activity is lower than that of the silica–aluminas,

Resoc-g 10.5 77.5 12.0 8.1 75.7 16.2 7.4 74.7 17.9

Table 9. Effect of the catalyst type on the product distribution during PP pyrolysis at 380 oC

Xie et al., 2008 have reported catalytic cracking of polypropylene (PP) over MCM-41 modified by Zr and Mo. The relationship among structure, acidity and catalytic activity of Zr–Mo-MCM-41 was studied. The results showed that Zr–Mo-MCM-41 exhibited high activity for the cracking of PP and good selectivity for producing liquid hydrocarbons of higher carbon numbers. The results were compared with those obtained over HZSM-5, SiO2–Al2O3 and other MCM-41 mesoporous molecular sieves. For the catalytic cracking of PP, Mo enhances the selectivity to high carbon number hydrocarbons and Zr enhances the acidity of catalyst and results in the increasing cracking conversion of PP. Zr–Mo-MCM-41 using Zr(SO4)2 as Zr source is of the best catalytic activity and selectivity to high carbon number hydrocarbon, which means that Zr–Mo-MCM-41 will probably become good

> Conversion (%)

HZSM-5 400 27.1 50.2 49.8 SiO2-Al2O3 400 25.8 60.3 39.7 Thermal cracking 400 30.4 76.2 23.8 Si-MCM-41 400 39.6 81.4 18.9 Mo-MCM-41 400 57.5 90.0 10.0 Zr-Mo-MCM-41 400 98.6 92.0 8.0 Zr-Mo-MCM-41 380 65.4 81.9 18.1 Zr-Mo-MCM-41 390 84.3 87.7 12.3 Zr-Mo-MCM-41 410 99.6 89.0 11.0 Table 10. Catalytic activities of different catalyst in PP pyrolysis for 30 min at catalyst/

Liquid yield (%)

Gas yield (%)

Reaction time = 12 min Reaction time = 24 min Reaction time = 72 min

5.2 78.6 16.2 4.9 72.2 22.9 6.7 70.3 23.0

13.2 81.4 5.4 11.2 81.5 7.3 10.2 82.7 7.1

6.1 77.8 16.1 5.8 75.8 18.4 6.7 76.2 17.1

8.9 81.0 10.1 8.6 78.1 13.3 9.5 78.8 11.7

+ gas oil

Gases Gasoline Diesel

+ gas oil

Gases Gasoline Diesel

this can be compensated by increasing the catalyst to PP ratio.

Gases Gasoline Diesel

Si–Al 13%

H-USY 500

H-USY 712

H-USY 760

(Cardona and Corma, 2000).

potential catalysts for the cracking of PP (Table 10).

(oC)

Catalyst Temperature

PP = 0.01 (Xie et al., 2008).

Catalyst Cumulative selectivity (%)

+ gas oil

Panda et al., 2011 investigated catalytic performances of kaoline and silica alumina in the thermal degradation of polypropylene. Polypropylene was cracked thermally and catalytically in the presence of kaoline and silica alumina in a semi batch reactor in the temperature range 400–550ºC in order to obtain suitable liquid fuels. It was observed that up to 450ºC thermal cracking temperature, the major product of pyrolysis was liquid oil and the major product at other higher temperatures (475–550ºC) are viscous liquid or wax and the highest yield of pyrolysis product is 82.85% by weight at 500ºC. Use of kaoline and silica alumina decreased the reaction time and increased the yield of liquid fraction. Again the major pyrolysis product in catalytic pyrolysis at all temperatures was low viscous liquid oil. Silica alumina was found better as compared to kaoline in liquid yield and in reducing the reaction temperature. The maximum oil yield using silica alumina and kaoline catalyst are 91% and 89.5% respectively. On the basis of the obtained results hypothetical continuous process of waste polypropylene plastics processing for engine fuel production can be presented (Table 11).

In conclusion, catalytic pyrolysis of PP reduces environmental impacts, also the time of recycling and results in very useful products, with potential use as fuel replacements.


Table 11. Pyrolysis of PP in optimum conditions (Panda and Singh, 2011).
