Influence of cobalt precursors on the characteristics and catalytic properties of MCM-41 supported cobalt catalysts for carbon monoxide hydrogenation

Abstract

Co/MCM-41 catalysts were prepared using the incipient wetness impregnation technique with aqueous solutions of different cobalt compounds such as cobalt nitrate, cobalt chloride, cobalt acetate, and cobalt acetylacetonate. MCM-41 is known to have a restricted pore structure; however, using organic precursors such as cobalt acetate and cobalt acetylacetonate resulted in very small cobalt oxide particles that could not be detected by XRD even for a Co loading as high as 8 wt%. These Co particles were small enough to fit into the pores of MCM-41. However, they were found to chemisorb H2 and CO in only relatively small amounts and to have low activities for CO hydrogenation probably due to the formation of cobalt silicates. Use of cobalt chloride resulted in very large cobalt particles/clusters and/or residual Cl- blocking active sites, consequently very small active surface area was measurable. Use of cobalt nitrate resulted in a number of small cobalt particles dispersed throughout MCM-41 and some larger particles located on the external surface of MCM-41. Cobalt nitrate appeared to be the best precursor for preparing high activity MCM-41-supported Co FTS catalysts. Furthermore, the metal-support interaction in supported Co Fischer-Tropsch catalysts depends strongly on the metal particle size and pore size of the supports. The properties of Co/MCM-41 catalysts prepared from different Co precursors were then compared to high surface area amorphous Co/SiO2 catalysts with similar support pore size. High surface area SiO2 showed similar metal-support interaction to MCM-41-supported catalysts as shown by similar TPR profiles. High activities for Co/MCM-41 catalysts were due primarily to higher Co dispersion in mesoporous structure of MCM-41 than in amorphous SiO2. Using organic cobalt precursors such as cobalt acetate and cobalt acetylacetonate and narrow pore supports resulted in very small cobalt particles deposited in the pores, leading to stronger metal-support interaction. The results suggest that there is a need for a balance between dispersion-enhancing, metal-support interaction and loss of metallic Co due to metal-support compound formation in order to obtain high activity supported Co catalysts.