**2. Chemical recycling of poly(ethylene terephthalate) PET**

PET is a polyester with functional ester groups that can be cleaved by some reagents, such as water (hydrolysis), alcohols (alcoholysis), acids (acidolysis), glycols (glycolysis), and amines (aminolysis). Thus, chemical recycling processes for PET are divided as follows: (i) Hydrolysis, (ii) Glycolysis, (iii) Methanolysis and (iv) other processes (Scheme 2). According to the reagent used different products are obtained (Karayannidis and Achilias, 2007; Karayannidis et al. 2006; Karayannidis et al., 2005; Karayannidis et al. 2002; Kosmidis et al., 2001). The different process options for chemical recycling of PET waste may be categorized as follows: (i) regeneration of base monomers (methanolysis for dimethyl terephthalate (DMT) and hydrolysis for producing pure Terephthalic acid (TPA) and ethylene glycol (EG)); (ii) conversion into oligomers (glycolysis or solvolysis); (iii) use of glycolyzed waste for value-added products; (iv) conversion into speciality chemicals by aminolysis or ammonolysis; (v) conversion into speciality intermediates for use in plastics and coatings.

Scheme 2. Chemical recycling techniques of PET.

Recent Advances in the Chemical Recycling

catalytic cracking (FCC) commercial catalysts.

Scheirs and Kaminsky, 2006 and in Achilias et al., 2006.

(Achilias et al., 2007).

**3.2 Pyrolysis** 

presented in section 5.

of Polymers (PP, PS, LDPE, HDPE, PVC, PC, Nylon, PMMA) 7

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

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

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

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

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

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

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, 1998; Karayannidis and Achilias, 2007).
