Effect of extra-lattice aluminium species on the activity, selectivity and stability of acid zeolites in the liquid phase isomerisation of α-pinene Original Research Article

The liquid phase isomerisation of α-pinene was studied on a number of zeolite catalysts at 150°C. The primary reaction products were camphene, tricyclene and p-menthadienes. Catalyst samples with lattice Si/Al ratios ranging from 1.46 to ca. 200 were prepared by hydrothermal treatment of zeolites Y and X. The nature of extra-framework aluminium species was investigated by27Al-MAS-NMR spectroscopy. For high values of NAl (number of framework aluminium atoms per unit cell) extra-lattice aluminium (EFAL) species have a promoting effect on the catalytic activity. On the other hand, for the highly dealuminated samples the catalytic activity decreases with NAl. However it does not seem to become zero for zero Brønsted acidity, suggesting that Lewis acid sites are also active on α-pinene isomerisation. Selectivity towards camphene is also strongly affected by EFAL species, decreasing at high values of NAl and increasing for the catalyst samples with high EFAL content. The different distribution of EFAL species in the catalysts prepared at different temperatures would cause this effect on selectivity.