Dicyclopentadiene → 2,3,3a,4-Tetrahydro-1H-indene

A reaction is carried out with a system consisting of one decomposition reactor, one Diels Alder reactor and five distillation columns. A mixture of a starting dicyclopentadiene material containing tetrahydroindene and a solvent (weight ratio of 2:5) is continuously fed to the decomposition reactor, and a reaction is carried out at the same temperature, pressure, reaction time and quantity of charged solvent as in Example 4. The ratio of tetrahydroindene/dicyclopentadiene (by weight) in the liquid starting material is adjusted to 5/100. A part of the liquid phase in the decomposition reactor is continuously discharged out of the system so as to keep the constant quantity of liquid in the decomposition reactor. The gaseous reaction product formed in the decomposition reactor is recovered, fed to the first distillation column (distillation "b1 "), and rectified at the same column top pressure, column bottom pressure, and reflux ratio as in Example 4. Fraction (B-1) composed mainly of cyclopentadiene is obtained from the column top, and fraction (B-2) composed mainly of dicyclopentadiene and tetrahydroindene is obtained from the column bottom. The fraction (B-2) is sent to the second distillation column (distillatior "b2 "), and rectified at the same column top pressure, column bottom pressure and reflux ratio as in Example 4. Fraction (B-4) composed mainly of tetrahydroindene is obtained from the column top, and fraction (B-3) composed mainly of dicyclopentadiene is obtained from the column bottom. The fraction (B-3) is recycled into the decomposition reactor, while the fraction (B-4) is discharged out of the system.