Catalytic properties of alkali metal-graphite intercalation compounds

Abstract

Identities, as well as purities, of synthesized alkali metal-graphite intercalation compounds were confirmed by X-ray diffraction methods. Second-stage compounds (MC(,24), M = K, Rb, or Cs) adsorb large amounts of nitrogen and hydrogen at -196(DEGREES)C via penetration of gaseous molecules. By contrast, the densely packed alkali metal layers in the corresponding first-stage compounds (MC(,8)) restrict the penetration of gaseous molecules. For ethylene hydrogenation at 40-70(DEGREES)C, catalytic activities increase from KC(,n) to CsC(,n) (n = 8 or 24), and activation energies decrease from KC(,n) to CsC(,n). Due to the high mobilities of alkali metal species in MC(,24), the latter have higher catalytic activities than do the corresponding MC(,8) compounds. Although exposure to large amounts of air deactivates these catalysts, addition of small amounts of air increases their activities to various extents. Based on this poisoning study, these intercalates can be divided into two groups: the first group contains KC(,8), KC(,24), and RbC(,8), and the second group contains RbC(,24), CsC(,8), and CsC(,24).For carbon monoxide hydrogenation, both catalytic activities and product distributions vary with CO conversion. Over KC(,8), KC(,24), and RbC(,8), maximum activities are quite high at 300(DEGREES)C, and ethane dominates the hydrocarbon products, whereas for RbC(,24), RbC(,36), CsC(,8), and CsC(,24), maximum activities are relatively low and large amounts of methane are formed. Both activities and product distributions are related to the strengths of metal-graphite interactions. Contrary to the behavior of transition metal catalysts, decreasing amounts of methane are produced with increasing reaction temperature. 1-Butene isomerization over these compounds proceeds via a base-catalyzed mechanism, as confirmed by high initial cis/trans product ratios. Activation energies over MC(,8), which depend on basicities of the graphitic anions, decrease from KC(,8) to CsC(,8). Again, MC(,24) has higher activity than the corresponding MC(,8) due to the greater mobilities of the alkali metal species. Negative temperature coefficients of activities for MC(,24) are related to the change of surface. The same two catalyst groups established for ethylene hydrogenation and CO hydrogenation may be identified by an O(,2) poisoning study for isomerization.