Changes in the catalytic behaviors of the Ni/Y-zeolite catalysts for CO hydrogenation were investigated with the distribution of nickel metal inside or outside of the zeolite.
All the catalysts were prepared by the conventional ion-exchange method. Both the degree of reduction and the distribution of nickel metal were measured quantitatively by using TPR/TPO experiments and were confirmed by TEM and FMR. The sizes of nickel crystallites were determined from X-ray line broadening, TEM, and hydrogenation chemisorption.
CO hydrogenation reaction was performed in a differential reactor operating at atmospheric pressure, temperature of 270℃, and $H_2$/CO ratio of 3. Nickel metal distribution affected the activity and product distribution significantly. With increasing fraction of nickel metal inside the zeolite the specific activity continued to decrease, while the production of long chain and olefinic hydrocarbons was enhanced significantly. Especially when all the nickel metals were restricted within the pores of the zeolite, a bimodal hydrocarbon product distribution of $C_1$ and $C_4$ was observed. This was ascribed to the dimerization of the primarily formed ethylene to butene on the active species of $Ni^+$.
