Hydrogen Transfer in Asphaltenes and Bitumen at 250 °C

Abstract

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 10 18 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.

Origin of Free Radical Persistence in Asphaltenes: Cage Effect and Steric Protection

“…In all reactions, 65–75% of the products formed by 1,2-dihydronaphthalene were heavier products. Dimerization products with the molecular formulas C 20 H 20 and C 20 H 22 were detected during gas chromatography, which was consistent with previous observations . The recovery efficiency of the dimerization products by methanol extraction was not known.…”

“…The 1,2-dihydronaphthalene is capable of donating hydrogen to form naphthalene as a product, and it is capable of accepting hydrogen to form tetralin as a product. Both reactions are thermodynamically favorable at 250 °C, and this reaction chemistry has been used before to study hydrogen transfer in asphaltenes . It was reasoned that by varying the concentration of 1,2-dihydronaphthalene, it would be possible to obtain an estimate of caging protection, as well as obtain an estimate of the direction of hydrogen transfer as the free radicals in the asphaltenes become “un-caged”.…”

“…The first control experiment involved the self-reaction of 1,2-dihydronaphthalene. The main products from self-reaction were known; conversion during self-reaction was around 20%. It was necessary to determine whether any of the self-reaction products formed persistent free radicals, such as was observed in the self-reaction of indene .…”

“…The recovered material was then extracted with methanol to recover unreacted 1,2-dihydronaphthalene and the reaction products from the conversion of asphaltenes with 1,2-dihydronaphthalene. A discussion on the solvent selection and procedure was previously reported 17 and will be summarized here. A large excess of methanol (100 mL) was added to the reaction product, which caused the lighter products to partition between the methanol and asphaltenes-rich phases.…”

Olefin Saturation Using Asphaltenes As a Hydrogen Source

“…The formation of hexyltetralin following on alkylation is consistent with the reaction thermodynamics. Although the reaction of 1,2-dihydronaphthalene to either tetralin or naphthalene is thermodynamically favorable, 14 it is unlikely that a radical intermediate on either reaction pathway would revert to the thermodynamically less favorable dihydronaphthalene. The formation of dodecene as opposed to dodecane is more likely a consequence of hydrogen availability.…”