Role of metal halides in coke formation during bitumen upgrading

•Acidic metal bromides increased solid yield during conversion of Athabasca vacuum residue at 400°C for 1h from 2.5wt% to 4–10wt%.•Halogen incorporation in the organic liquid after thermal conversion with 1wt% acidic metal bromides was 3–7μmol/g.•Increase solids yield and transfer of halogen by acidic metal bromides was suppressed, but not eliminated by the addition of a base.•Thermal cracking in the presence of a base, Mg(OH)2, decreased overall solid yield.