Nickel catalysts site densities

Deactivation parameters obtained by plotting ln[(l — a) a)] versus time are listed in Table XIX for a number of nickel and nickel bimetallic catalysts. The fact that these plots were generally linear confirms that these data are fitted well by this deactivation model. These data, which include initial site densities for sulfur adsorption, deactivation rate constants, and breakthrough times for poisoning by 1-ppm H2S at a space velocity of 3000 hr-1 provide meaningful comparisons of sulfur resistance and catalyst life for both unsupported and supported catalysts. Table XIX shows that the... 

Comparison of the site densities from Table XIX with metal areas determined from H2 adsorption provides important insights into the nature of H2S adsorption on these catalysts. For example, the sulfur site density of 213 /tmol/g compared to the metal site density of 182 /rmol/g (from H2 adsorption) for 14% Ni/Al203 is equivalent to S/Nis = 0.6, in reasonable agreement with the earlier discussed studies (Section III,C) which show values of 0.5-0.8 and consistent with the value 0.6 determined for pure unsupported Ni. However, in the case of a typical molybdenum-containing catalyst, e.g., 10% Ni/20% Mo/A1203, the sulfur site density and H2 uptake are 693 and 72 /imol/g, respectively (S/Nis = 4.8), providing evidence that a considerable amount of sulfur adsorbs on molybdenum oxide sites which do not adsorb H2 a similar behavior is also observed for Raney Ni and nickel-boride catalysts. 

It can be seen that the activities of the amorphous alloys are lower than those of the polycrystalline catalysts. Formation of the corresponding diol was not observed on the amorphous catalysts, while the crystalline catalysts either produced the diol selectively, or a mixture of the diol and the hydroxy ketone was formed. The fundamental reason for the lower activity and higher selectivity of the amorphous alloys is their rather small surface area. Of the amorphous alloys studied, Ni-B and Ni-P alloy powders prepared by chemical reduction exhibited higher activities than those of Ni-P alloys prepared by electrolytic reduction or rapid quenching. This difference in activity can be attributed to an oxide layer covering the surface of m-P foils [Ij. It is necessary to point out, however, that the comparison of activities is based on unit catalyst weight. Obviously, this comparison does not take into account the real surface area of the nickel samples, nor active site densities. 

Supramolecular catalysis may also involve the combination of a host cavity and a metal active site as in the bis(diphenylphosphino)calix[4]arene nickel(II) complex 12.40 which acts as an efficient catalyst for ethylene and propylene polymerisation, and in tandem with zirconocene dichloride, for the formation of linear low-density polyethylene. In the latter case the complex gives very little branching - a significant advantage. The key to the effectiveness of the catalyst involves calixarene-induced changes in the bite angle at the Ni(II) centre, which is square planar in the active form of the catalyst.29... 

The Chen group [26] has recently synthesized nickel complex 31 (Figure 5.7) with an NHC-tethered phenanthroline ligand. They have hypothesized that the NHC donor would increase the electron density at the nickel center while the phenanthroline unit could easily dissociate one of the N donors (presumably the outer one) to create a vacant coordination site, thereby facilitating the catalysis. This compound has proven to be a good catalyst (1 mol% loading) for the coupling of aryl chlorides... 

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