Dimerization reactions of benzothiophene were performed using 10 wt% Al-2(SO4)(3) supported on porous silica gel as a solid acid catalyst under atmospheric or supercritical CO2 conditions, in order to purify naphthalene as well as to separate and recover benzothiophene(BT). BT was more selectively dimerized under supercritical CO2 conditions (100 degrees C, 100 atm) than in atmospheric octane solution at the same reaction temperature, allowing its selective dimerization even when mixed with naphthalene. Effects of reaction pressure, solvent, and reaction time on the dimerization selectivity were examined based on the analyses of recovered fractions and products. Five types of benzothiophene dimers were identified, being produced more selectively under the supercritical CO2 conditions. The dimers were easily converted to BT monomer by heating to around 250 degrees C, suggesting the possibility of recovery of BT from crude naphthalene.