Zeolites are now common catalytic materials in widespread use in the petroleum industry and belong to a wider group of aluminosilicates known as molecular sieves.165 Zeolites are porous solids with pore sizes around 2-10 A and are described by IUPAC as microporous. A typical example is shown in Fig. 14.2. Unlike the aluminosilicates that are present in common clays that have a layered structure, zeolites have rigid honeycomb-like pore structures that can survive many reactions (ion-exchange, hydrogenation, hydroxylation and de-hydroxylation)

without swelling, contraction and collapse of the pore structure. Zeolites are usually hydrated and contain water which is only weakly held. Zeolites are strong solid acids. They are well known for their ion exchange capability which allows the ‘doping’ of the structure by hetero atoms and, thus, their chemical modification. They are crystalline materials with AlO4 and SiO4 units tetrahedrally linked through oxygen anions. In recent times, phosphate groups have been introduced in an effort to generate larger pore systems. Both the atomic and pore structure is periodic and almost 200 zeolite structures have been detailed.166 Zeolites can be synthesised via hydrothermal condensation of cation precursors in the presence of an organic template167 but also occur naturally, and the most important natural zeolite is clinoptilolite.168

The general catalytic chemistry of zeolites is well reviewed169 and, in particular, the hydrocarbon cracking properties detailed.170 Zeolites have high active site densities due to the in balance of anionic charge between the AlO4 and SiO4 units.171 The first found use in petroleum refineries almost 50 years ago where their high activity to hydrocarbon cracking coupled to shape selectivity provided by their pore structure, allowed industrial chemists to achieve greater yields of useful petroleum products.172 The year 1972 saw the advent of a synthetic high silica zeolite catalyst to the energy industry, HZSM-5, that was able to produce aromatic, petroleum-type products from a wide range of hydrocarbon sources.173 This material has become the standard against which most pyrolysis catalysts are assessed. The wealth of data available that demonstrates the effectiveness of zeolite catalysts in pyrolysis is now considerable and, therefore, only some of the more recent data is reviewed here.

Zeolites have been widely applied to the catalytic pyrolysis for waste polymer treatments. Despite the advances in polymer recycling for re-use as construction, consumer and retail materials this can be expensive (shipping to low-cost economies for sorting, energy intensive, use of solvents, environmental issues, etc.), result in low-grade products as well as being technically difficult for some polymer types.174 The opportunity to develop thermolysis methods including pyrolysis (as well as hydrogenation and gasification) for energy generation using waste polymer has been reviewed by Mastellone et a/.175,176 The mechanisms of polymer thermal decomposition are now well accepted.177 Whilst all aluminium containing zeolites have good cracking capability, it is generally found that the larger pore zeolites such as zeolite-Y (0.74 nm) produce less gas and low molecular weight (hydrocarbons with three to six carbons) products and a higher relative content of aromatics (which increase the product octane number and ensures the smooth running of internal combustion engines and use as a transport fuel178) compared to the smaller pore systems such as ZMS-5 (0.55 nm) and zeolite-A (0.4 nm).41,179-182 The reason for the behaviour is apparently due to the increased number of acid sites on zeolite-Y type systems183 as well as the small pore size of ZMS-5 which limits the diffusion of larger hydrocarbon moieties during reaction.184

Zeolite catalysts have also been used for the pyrolysis of biomass and related materials. In a study of the pyrolysis of a synthetic bio-oil showed that HZMS-5 was a better catalyst for aromatic production than a range of other zeolite, transition metal and mesoporous catalysts.133 The catalyst completely removed water and oxygen from the bio-oil in the conditions used but the authors also found that all the catalysts studied increase gas production at the cost of liquid generation. Uzun and Sarioglu also found that zeolite catalysts decreased liquid yield whilst increasing the proportion of gas and this seems to be a general finding from many studies.124 As above for polymeric materials, these authors also found that zeolite-Y increased the aromatic content but did maintain the highest liquid yield.124 Carlson et al. found that ZMS-5 was the catalyst that provided greater aromatic quantise for a range of biomass materials (cellulose, cellobiose, glucose and xylitol) when compared to other zeolites and mesoporous silica.127 Aho et al. studied a range of zeolite-b catalysts over a range of Si:Al ratios of 25-300 and found that the increasing acidity resulted in both greater gas yields as well as the amount of coking.185 These authors also demonstrated that the catalyst was a prerequisite in generation of polyaromatics from biomass. Zeolite catalysts have been widely used for the catalytic pyrolysis of vegetable and plant oils. One of the first studies of HZSM-5 was for the pyrolysis of corn and peanut oil in 1979.186 A high aromatic yield was found with the product mixture being akin to a high-grade petrol (gasoline).186 Similar findings were later reported by Milne and co-workers.173 It is generally thought that the HZMS-5 catalyst is the most effective type of zeolite catalyst for the conversion of vegetable oils to quality fuel materials.187,188 Similar effectiveness was found for the conversion of palm oil.189,190