For many years it has been known that acid sites are required for catalytic cracking and in the work of Thomas these were thought to be predominantly formed at aluminium sites connected through oxygen ions to silica.145 These sites are known to promote carbonium ion mechanisms and are probably the major reason for lowering the pyrolysis temperature. Such sites are not present on alumina. However, activated alumina has become an important adsorbent and catalyst and contains both Lewis (electron pair accepting) and Brpnsted acid (releases H+) sites. It is made by thermal de-hydroxylation of Boehmite, an aluminium oxide hydroxide (y-AlO(OH)) and yields a highly porous powder of surface area > 200 m2/g. The product alumina is normally in the у or n crystal structure which are conducive to generating high surface areas.162 The main role of activated alumina catalysts appears to be de-hydroxylation of hydrocarbons.163 This has been shown for the catalytic pyrolysis of a series of vegetable oils where the products were essentially linear hydrocarbons containing no oxygen.164 Chang et al. have reported that alumina has poor cracking and hydrogenation ability, consequently, the yield of low molecular weight products from wood biomass is small.136 For a study of the effect of a catalyst on the products of thermal pyrolysis, Samolada et al. studied a range of catalysts on the treatment of a synthetic bio — oil.133 They found that whilst y-alumina had little cracking ability beyond that expected of thermal treatment it did, however, notably improve the quality of the pyrolysis oil (via de-hydroxylation). a-alumina (low surface area) used as a catalyst exhibited little of this improvement of the bio-oil material.