Types and Properties of Polymer Cracking Catalysts

A wide variety of catalysts have been found effective in promoting the decomposition of plastic materials Friedel-Crafts catalysts, acidic and basic solids, bifunctional solids, etc. Friedel-Crafts systems, mainly A1C13/HC1, were initially used as acid catalysts but they have now been replaced in most processes by solids with acid properties due to the corrosion and environmental problems they cause. 

The most common catalysts used in plastic cracking are acidic solids, mainly alumina, amorphous silica-alumina and zeolites. These materials are the catalysts typically used in the petroleum processing and petrochemical industries. They have very different textural and acid properties, which directly determine their catalytic activity and product selectivity. Thus, while the acidity of alumina is of Lewis type, both Brdnsted and Lewis acid sites may be present in amorphous silica-alumina and zeolites. This is an important factor because 

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

As a consequence of all these properties, zeolites can be considered to be exceptional catalysts which have replaced amorphous solids in many applications. However, for the catalytic cracking of polymeric wastes, zeolites may be disadvantageous due to the steric and diffusional problems that polymer molecules may have in accessing the zeolite micropores. These drawbacks can be 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 subjected to steric hindrances for the conversion of bulky substrates. 

Other interesting solids for the catalytic degradation of polymeric wastes are the various silica-based mesophases which have recently been discovered.4,5 These materials are characterized by the presence of ordered and regular pore systems and high surface areas, typically over 1000 m2 g l. The most common member of this family is MCM-41, which has a hexagonal array of uniform pores with diameters that can be tailored in the range 1.5-10 nm by varying the synthesis conditions. These mesoporous materials can be prepared with a wide 

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