In this paper, we have studied the oligomerization of ethylene with a liquid heptane solvent over bifunctional Ni catalysts in a continuous flow reactor. We have prepared an Al-containing KIT-6 silica that was used as a support after calcination in the temperature range of 300-900 A degrees C. The Ni/Al-KIT-6 catalysts had uniform mesopores with diameters in the range of 5.4-6.3 nm, excepting Ni/Al-KIT-6 (900). The calcination temperature of Al-KIT-6 support changed the surface acidity as well as the interaction of Ni2+ and acid sites for the Ni catalysts, as determined by temperature-programmed desorption of ammonia, temperature-programmed reduction, infrared spectroscopy after the adsorption of pyridine, solid-state Al-27 magic-angle spinning nuclear magnetic resonance spectroscopy, and X-ray adsorption spectroscopy. Among the tested catalysts, the Ni/Al-KIT-6 (300) showed the highest ethylene conversion because of the increased intimate contact between Ni2+ and acid sites. The strong interaction of Ni2+ species and the support is not effective in increasing active sites for ethylene conversion. The Ni/Al-KIT-6 catalysts produced internal linear C4 and C6 olefins with high selectivity. The Ni/Al-KIT-6 (300) had 2.2-6.1 times lower selectivities toward 2-ethyl-1-butene than other catalysts at similar ethylene conversions. The reaction product mixture showed that the Ni/Al-KIT-6 catalysts shifted the product distribution towards acid-catalyzed oligomerization/cracking/realkylation products (i.e. C3, C7, C7, and C8+ olefins) as the concentration of Bronsted acid sites increased. Among the tested catalysts, the Ni/Al-KIT-6 (300) showed the highest yield of C4 and C6 olefins (78.3%).