A new concept on the direct synthesis of middle iso-paraffins through the modified supercritical Fischer-Tropsch (FT) reaction was proposed and experimentally demonstrated with the combination of CO/SiO2 and palladium-supported beta zeolite (Pd/beta) catalysts both in one-stage and two-stage fixed-bed reaction systems. Depending on the reaction conditions, the selectivity of C-4(+) iso-paraffins mostly in gasoline range was 60-80% due to hydrocracking and hydro-isomerization function of Pd/beta. Irrespective of reaction conditions, the hydrocarbons produced from FT reaction were preferentially hydro-converted over Pd/beta catalyst while the supercritical solvent of n-hexane could only be slightly hydro-converted under severe conditions. The production of iso-paraffin in two-stage process was tested for 100 It without observable deactivation by feeding additional hydrogen in lower reactor, which is ascribed to the inhibition of coke deposition over Pd/beta as revealed from TG analysis of the used catalysts. (c) 2005 Elsevier B.V. All rights reserved.