Selective production of middle distillate (C-10-C-20) from synthesis gas (CO + H-2) was carried out in a dual-bed reactor. Co-based catalysts were used in the first-bed reactor to produce wax (C21+) from synthesis gas, and Pd-loaded solid acid catalysts were used in the second-bed reactor to produce middle distillate from wax through hydrocracking. Co/TiO2 catalyst in the first-bed reactor produced more than 35 wt% middle distillate and more than 20 wt% wax. These products served as a suitable feed for the production of middle distillate through hydrocracking in the second-bed reactor. Pd-loaded solid acid catalysts used in the second-bed reactor retained three types of acid sites (weak-, medium-, and strong-acid sites). Correlations between medium acidity and catalytic activity of Pd-loaded solid acid catalyst revealed that wax conversion and increment of middle distillate selectivity in the second-bed reactor were increased with increasing medium acidity of the catalyst. Total acidity of Pd-loaded solid acid catalyst was increased with increasing medium acidity of the catalyst. Thus, the medium acidity (total acidity) of Pd-loaded solid acid catalyst served as a crucial factor in hydrocracking of wax. Among the second-bed catalysts, the Pd/mesoporous alumina (Pd/MA) catalyst with the highest medium acidity (total acidity) showed the best catalytic performance (ca. 83% wax conversion and ca. 15 wt% increment of middle distillate selectivity). (c) 2008 Elsevier B.V. All rights reserved.