Hydrogen Transfer in Asphaltenes and Bitumen at 250 °C

Hydrogen transfer in asphaltenes and bitumen was studied at 250 °C. Use was made of dihydronaphthalene as probe molecule, which has both hydrogen donor and hydrogen acceptor properties. Tetralin and naphthalene, which are the products from hydrogen disproportionation of dihydronaphthalene, were unreactive as hydrogen donor and hydrogen acceptor in control reactions with asphaltenes. The asphaltenes fraction was a net hydrogen acceptor, whereas bitumen was a net hydrogen donor during prolonged conversion with dihydronaphthalene. The free radical content of asphaltenes and bitumen (of the order of 1018 spins/g in the feed) changed during thermal conversion and dihydronaphthalene affected these changes only to a minor extent. In the case of bitumen, there was a decrease in the free radical content of the product irrespective of whether dihydronaphthalene was present or not. Yet, despite the presence of free radicals in asphaltenes and bitumen, the reaction rate of dihydronaphthalene was suppressed compared to the self-reaction of dihydronaphthalene. Dihydronaphthalene reacted by molecule-induced homolysis to produce free radical intermediates; the initial reaction rate was around 1.6 × 10–3 mol·h–1·g–1. It was found that hydrogen donation by the free radical intermediates was faster than hydrogen abstraction, so that naphthalene was the most abundant product from self-reaction and that the heavier products formed by free radical addition were hydrogen enriched. Although formation of tetralin and naphthalene was thermodynamically favored, conversion was not equilibrium controlled and stoichiometric limitations were imposed by hydrogen disproportionation. Product selectivity during the self-reaction of dihydronaphthalene, and the decrease in reaction rate of dihydronaphthalene in the presence of free radical containing feed, both challenged the assumption that hydrogen abstraction by free radicals did not require activation energy. The work demonstrated that molecule-induced homolysis readily takes place at 250 °C and that oilsands derived asphaltenes and bitumen are reactive for hydrogen transfer at these conditions.