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

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 by ²⁷Al-MAS-NMR spectroscopy. For high values of N_Al (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 N_Al. 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 N_Al 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.