Whilst the zeolite catalysts described above are highly effective and form the basis of the majority of large scale process plants (see below for FCC catalysts), scientists are still searching for catalysts to improve the efficiency of catalytic pyrolysis. Probably, the major driving force has been the need to reduce gas yields and, hence, improve liquid content. In order to generate more efficient processes, a further type of molecular sieve materials has begun to be more extensively studied. These are mesoporous materials (normally silicates) that were first detailed by researchers at the Mobil Research and Development Corporation in New Jersey.191 These are now heavily researched for applications in many fields and readers are referred to a recent excellent collection of papers that provide a comprehensive review of the field.192 A typical example is shown in Fig. 14.3. The first mesoporous materials were given the abbreviation MCM (Mobil Composition of Matter) and, like zeolites, consisted of tetrahedral silica-oxygen linkages through which ran regularly arranged (periodic), uniform sized pores. Like zeolites, mesoporous materials are formed from organic templating or

structure-directing molecules but in mesoporous solid synthesis chemistry it is generally thought that the templating species are aggregates of the organics into micellar forms and this results in pore sizes that are around ten times (i. e. in the range of a few nanometre) that of the general zeolitic microporous solids (although much recent work has been carried out to extend this). Where the mesoporous materials differ is that the inorganic framework is amorphous and has no crystalline order and periodicity only arises from the pore structures. The most common pore arrangement is an hexagonal honeycomb arrangement which is more robust than a lamellar and cubic form. Materials that follow the Mobil synthesis route and have the hexagonal structure are normally described as MCM-41 and it is this material (and analogues) that dominates the mesoporous catalytic pyrolysis literature. However, the synthesis chemistry is well-developed and routes to complex combinations of macroporous-mesoporous structures,193 thin films194 and complex particle shapes195 as well as precise control of pore size are well documented.196

One of the important steps in providing active mesoporous catalysts is the incorporation of aluminium to provide acid sites and the usual material investigated for catalytic pyrolysis is Al-MCM-41.197 However, more acidic samples gave greater amounts of coke and gas products suggesting that an optimum aluminium content exists.197 These authors also report a lack of hydrothermal robustness of these materials and that would have a major effect on the industrial use of these materials.197 The relative hydrothermal stability of mesoporous silicates compared to zeolites and amorphous aluminosilicates for hydrocarbon cracking is well known.198 However, they do show good activity for the production of fuel oils from palm oil.199 Triantafyllidis and co-workers have shown that the nature of the acid site in Al-MCM-41 (since both Brpnsted and Lewis acid sites are formed) can have effects on the molecular distribution of the resultant fuel oils.200 One of the major reasons for the use of large pore systems is the possibility of improving the yield of higher molecular weight products and reducing the gas products. The rationale for the use of mesoporous systems (compared to microporous systems) is that the larger pore systems could allow diffusion and reaction of larger moieties and there is evidence in the MCM system for this pore size effect.201

Larger pore mesoporous systems include SBA-15, which is known to be more hydrothermal robust (because of thicker pore walls than MCM-41) and can be readily synthesised with alumina content.202 Qiang et al. have used various SBA-15 catalysts to study pyrolysis of sawdust.203 As might be expected, it was found that Al-SBA-15 significantly outperformed SBA containing no aluminium analogues and that catalytic activity improved with the amount of aluminium added.203 Aguado and co-workers have shown that Al-SBA-15 can also be used for the catalytic pyrolysis of polyolefins and shows very promising characteristics.204 In particular, hydrothermally stabilised SBAs outperformed ZSM-5 and this was attributed directly to the larger pore size which reduced diffusional limitations (as described above). To support the conclusion that SBA-15 may be a better pyrolysis catalyst than MCM-14, Cao et al. have shown that SBA-15 measurably improves the product fuel quality compared to MCM type materials.205