Rearrangement of Tetrahydrotricyclopentadiene Using Acidic Ionic Liquid: Synthesis of Diamondoid Fuel

As a work successive to AlCl 3 catalytic isomerization of tetrahydrotricyclopentadiene (THTCPD) [ Wang, L.; Zhang, X.; Zou, J.-J.; Han, H.; Li, Y; Wang, L. Energy Fuels 2009, 23 (5), 2383−2388], the reaction using chloroaluminate ionic liquid (IL) was investigated. It is found that IL catalysis gives totally different product distribution. The endo -cycloproyl fragments of THTCPD that cannot be isomerized by AlCl 3 catalysis are transferred to exo conformation easily. Furthermore, the hydrocarbons are transferred to diamondoids, including methyl-1,2-tetramethyleneadamantane, methyl-diethyl-adamantane, and methyl-diamantane via skeletal rearrangement, with the first diamondoid as the primary and dominant product (selectivity > 80%). The IL shows much higher activity than superacid CF 3 SO 3 H, attributed to the synergetic effect of strong acidity and the novel solvent environment. Increasing the temperature, IL dosage, and AlCl 3 in IL can promote the rearrangement rate with a slight decrease in the selectivity of methyl-1,2-tetramethyleneadamantane. The presence of trace water in IL forms superacid and induces considerable cracking byproducts, and solvent typically used in rearrangement decreases the reaction rate. The diamondoid-based product shows a low freezing point and high hydrogen content and, thus, is superior as a high-energy-density fuel. This work may open a door for facile synthesis of diamondoid fuel.