Role of Brønsted acid centres for alkene double bond migration over alumina at temperatures above 450 K

Abstract

Water was added to alumina samples following their activation in vacuo at both 723 and 1023 K. Reactions of [²H₀]propene and of CD₂CH—CH₃ were then studied on these catalysts. Adsorption of water (2.7 × 10¹⁸ molecule m^–2) at ∼300 K is molecular and serves only slightly to poison catalytic activity at ∼300 K. Adsorption of water at > 453 K occurs dissociatively and completely poisons those sites on the fresh catalyst surface which promote reactions at ∼300 K. Reaction of propene occurs on such a water poisoned catalyst at ∼483 K and proceeds with incorporation of D⁺(from D₂O) through an associative mechanism involving carbonium ions, in contrast to a dissociative mechanism at a reaction temperature of 300 K on an unpoisoned catalyst. Water “added-back” to activated alumina at > 453 K is therefore effective in producing Brϕnsted acid centres on the surface and it is suggested that residual hydroxyls, those left following activation, can also participate in carbonium ion formation from propene at > 453 K. Therefore, not only is the activity of alumina strongly influenced by its activation temperature but it is also determined by the temperature used for subsequent alkene catalysis.