Nickel catalysts sulfur addition

Catalytic hydrogenation is mostly used to convert C—C triple bonds into C C double bonds and alkenes into alkanes or to replace allylic or benzylic hetero atoms by hydrogen (H. Kropf, 1980). Simple theory postulates cis- or syn-addition of hydrogen to the C—C triple or double bond with heterogeneous (R. L. Augustine, 1965, 1968, 1976 P. N. Rylander, 1979) and homogeneous (A. J. Birch, 1976) catalysts. Sulfur functions can be removed with reducing metals, e. g. with Raney nickel (G. R. Pettit, 1962 A). Heteroaromatic systems may be reduced with the aid of ruthenium on carbon. 

This solution Is heated to 65°C and barium hydroxide added in quantity sufficient to make the concentration of the barium hydroxide 0.2 mol/liter. The solution is agitated and maintained at 65°C for 6 hours after the addition of the barium hydroxide. It is then cooled and neutralized to a pH of 6.8 with sulfuric acid. The precipitated barium sulfate is filtered out. A quantity of activated supported nickel catalyst containing 5 g of nickel is added. 

It should be emphasized here that even though sulfur tolerances (steady-state activities in the presence of sulfur impurities), of promoted and unpromoted nickel catalysts are extremely low, their sulfur resistances (rates of activity loss in the presence of sulfur impurities), can vary greatly with catalyst configuration, composition, and support. Thus it may be possible to extend significantly nickel catalyst life in the presence of sulfur poisons through addition of promoters or use of novel supports (23,99,100,113, 114, 161, 194, 225-227). This is in fact the basis of two recent patents (225, 226). 

Figure 20.11. Effects of different level of sulfur addition on the quantity of total CLA formed in soybean oil during hydrogenation with 0.15% nickel catalyst. The hydrogenation was performed under the condition of hydrogen pressure 0.5 kg/cm2, reaction temperature 220°C, and agitation rate 500rpm. Source Ju and Jung, 2003.

The colloidal nickel catalyst cannot be removed by conventional filtration techniques nor have effective means of deactivation by poisoning been found. Ziegler claims that addition of colloidal iron will poison the nickel catalyst. The use of iron and other potential nickel poisons has been studied in some detail. Salts of Cd, Cu, Cr, Fe, Hg, Se, V, and Zn along with phenylacetylene and sulfur dichloride have been tested as nickel deaotivators. Iron, cadmium, and copper salts seemed effective in limiting alkylation between olefins and triethylaluminum... 

However, in some cases partial catalyst poisoning is desired, for example to lower the catalyst activity or to influence the selectivity. A well-known example is the addition of ppm quantities of H2S in catalytic reforming with nickel catalysts. Compared to platinum, nickel has a higher hydrogenolysis activity, which leads to formation of gases and coke. Sulfur selectively poisons the most active hydrogenolysis centers and thus drastically influences the selectivity towards the desired isomerization reactions. 

Dithioketals and dithioacetals can be induced to undergo reductive desulfurization on heating alcoholic solutions of those dithio-derivatives in the presence of moist Raney nickel (see, e.g., page 746). These sulfur compounds are prepared by treating the aldehyde or ketone with 1,2-ethanedithiol (CAUTION STENCH) in the presence of an add catalyst (an addition to the carbon of the carbonyl with loss of water, a process that will be discussed at length, vide infra) (Scheme 9.13). 

Methane or natural gas steam reforming performed on an industrial scale over nickel catalysts is described above. Nickel catalysts are also used in large scale productions for the partial oxidation and autothermal reforming of natural gas [216]. They contain between 7 and 80 wt.% nickel on various carriers such as a-alumina, magnesia, zirconia and spinels. Calcium aluminate, 10-13 wt.%, frequently serves as a binder and a combination of up to 7 wt.% potassium and up to 16 wt.% silica is added to suppress coke formation, which is a major issue for nickel catalysts under conditions of partial oxidation [216]. Novel formulations contain 10 wt.% nickel and 5 wt.% sulfur on an alumina carrier [217]. The reaction is usually performed at temperatures exceeding 700 °C. Perovskite catalysts based upon nickel and lanthanide allow high nickel dispersion, which reduces coke formation. In addition, the perovskite structure is temperature resistant. 

Due to sluggish reactivity of aryl and vinyl halides in nucleophiUc substitution reactions, the formation of sulfur-carbon(sp ) bonds is typically carried out using transition metal catalysis [22-27]. While the field is dominated by the use of palladium, copper, and nickel catalysts, considerable advances have been made using more abundant metal catalysts such as iron. Additionally, a number of transition metal-fiee approaches have been developed for the formation of sulfur-carbon(sp ) bonds. The following sections will highlight representative examples of C—S bond forming reactions. 

Durability of SOFCs is not solely related to thermal mismatch issues. The electrodes suffer a shong poisoning effect from sulfur (in the parts-per-billion range), requiring the use of sulfur-free fuels. Additionally, anode oxidation of nickel catalysts can decrease performance and will do so rapidly if the SOFC is operated below 0.5 V. This can be a problem in stack operation if not every cell is monitored, since individual cells in series... 

Other Specialty Chemicals. In fuel-ceU technology, nickel oxide cathodes have been demonstrated for the conversion of synthesis gas and the generation of electricity (199) (see Fuel cells). Nickel salts have been proposed as additions to water-flood tertiary cmde-oil recovery systems (see Petroleum, ENHANCED oil recovery). The salt forms nickel sulfide, which is an oxidation catalyst for H2S, and provides corrosion protection for downweU equipment. Sulfur-containing nickel complexes have been used to limit the oxidative deterioration of solvent-refined mineral oils (200). 

Metals and alloys, the principal industrial metalhc catalysts, are found in periodic group TII, which are transition elements with almost-completed 3d, 4d, and 5d electronic orbits. According to theory, electrons from adsorbed molecules can fill the vacancies in the incomplete shells and thus make a chemical bond. What happens subsequently depends on the operating conditions. Platinum, palladium, and nickel form both hydrides and oxides they are effective in hydrogenation (vegetable oils) and oxidation (ammonia or sulfur dioxide). Alloys do not always have catalytic properties intermediate between those of the component metals, since the surface condition may be different from the bulk and catalysis is a function of the surface condition. Addition of some rhenium to Pt/AlgO permits the use of lower temperatures and slows the deactivation rate. The mechanism of catalysis by alloys is still controversial in many instances. 

---