Isomerization of exo-tetrahydrodicyclopentadiene to adamantane using an acidity-adjustable chloroaluminate ionic liquid

Adamantane (ADM) was synthesized by isomerization of exo-tetrahydrodicyclopentadiene (exo-THDCPD) using chloroaluminate ionic liquids (ILs) as the catalyst. The yield of ADM was optimized to obtain 21.9% under the conditions of 70 °C for 6 h and pyridine hydrochloride/aluminum trichloride catalyst (PHC/AlCl3) at an AlCl3 mole fraction of 0.65. Under these conditions, the selectivity of ADM was 66% and the product mixture was easily recovered from the IL catalyst phase. It is proposed that endo-/exo-isomerization occurs via a carbocation mechanism leading to ADM, the main product, with ring-opening to decalin and other C10H18 by-products.