Nickel catalyst reoxidations

Alumina-zeolite supported nickel catalysts were investigated for the conversion of n-hexane and dewaxing of diesel oil fraction. The method of nickel incorporation influenced the susceptibility of nickel to reduction and catalyst activity. Investigated also was the effect of reoxidation temperature on the accessibility of the metallic surface. Differences in the activities of the catalysts can be attributed to the morphology of the deposits. 

A/iPW-hydroxybenzaldehyde is available and hydrogenation of (38) with Raney nickel catalyst reduces the double bond and the aromatic ring, leaving only reoxidation for a short synthesis of (34). The overall yield is 25%. 

XPS measurements demonstrated that loaded Ni is predominantly located between the layeres of the catalyst and little remains on the external surface.15) For sensitivity reasons, a sample with 1 wt% Ni-loading was used. Comparison of the Ni2p3/2 peak intensity in the catalyst with that in a reference sample (which was also 1% Ni-loaded KNb03 with almost the same BET surface area as that of K4Nb6017) has shown that the surface concentration of Ni in the former is about 100 times less than that of the reference sampled EXAFS spectra for 1 wt% Ni-loaded samples both before and after the reduction procedure, as well as for Ni and NiO as standards, indicated that after reduction by H2 at 500°C for 2 b the loaded Ni was completely reduced to the metallic state.15) Even after reoxidation by 02 at 200°C for 1 h, most of the Ni remained metallic. (By XPS, the Ni, which remained on the external surface, was found to be in the oxidized form.) The formation of metallic nickel on a 0.1 wt% Ni-loaded catalyst was also confirmed by ESR measurements.7 The appearance of an intense resonance line after the reduction and reoxidation indicates the formation of ferromagnetic metallic nickel in the sample. 

Normally, the nickel oxide in the catalyst is reduced to nickel and water by the hydrogen that is produced in the operation. In some cases the reduced nickel can be reoxidized to nickel oxide when large amounts of steam and small amounts of H2 are present47 ... 

Hydrosilicate formation is also in evidence in the Cu(II)-Si02 system. Via precipitation from a homogeneous solution one can obtain highly dispersed copper oxide on silica (cf. above, Fig. 9.10, where it should be noted that the Cu case is more complicated than the Mn one in that intermediate precipitation of basic salts can occur). Reaction to copper hydrosilicate is evident from temperature-programmed reduction. As shown in Fig. 9.12 the freshly dried catalyst exhibits reduction in two peaks, one due to Cu(II) (hydr)oxide and the other, at higher temperature, to Cu(II) hydrosilicate. Reoxidation of the metallic copper particles leads to Cu(II) oxide, and subsequent reduction proceeds therefore in one step. The water resulting from the reduction of the oxide does not produce significant amounts of copper hydrosilicate, in contrast to what usually happens in the case of nickel. 

It is well known that all sulfur compounds rapidly deactivate iron, cobalt and nickel Fischer-Tropsch catalysts. However, due to the efficiency of modem gas purification processes such as (he Lurgi Kectisol process, the sulfur level in synthesis gas can be reduced below 0.03 tng/mj. Tiiis level is tolerable and a constant synthesis gas conversion can be achieved [15], Iron catalysis which have been poisoned by sulfur are not readily reactivated. Only very thorouglt reoxidation by which all traces of sulfur are burnt away efficienily. followed by reduction, is effective [15,21J. 

The unusual oxidant nickel peroxide converts aromatic aldehydes into carboxylic acids at 30-60 °C after 1.5-3 h in 58-100% yields [934. The oxidation of aldehydes to acids by pure ruthenium tetroxide results in very low yields [940. On the contrary, potassium ruthenate, prepared in situ from ruthenium trichloride and potassium persulfate in water and used in catalytic amounts, leads to a 99% yield of m-nitrobenzoic acid at room temperature after 2 h. Another oxidant, iodosobenzene in the presence of tris(triphenylphosphine)ruthenium dichloride, converts benzaldehyde into benzoic acid in 96% yield at room temperature [785]. The same reaction with a 91% yield is accomplished by treatment of benzaldehyde with osmium tetroxide as a catalyst and cumene hydroperoxide as a reoxidant [1163]. 

To overcome the objectionable reoxidation of formaldehyde and decomposition at the temperature of the reaction zone in the oxidation of methane, it has been proposed to react the formaldehyde as fast as formed with some substance to give a compound more stable under the conditions of the reaction and thus to increase the yields obtainable. It is claimed 101 that a reaction between the newly formed formaldehyde and annnonia to form a more stable compound, hexamethylene-tetramine, is possible under certain conditions, so that the formaldehyde is saved from destruction and can be obtained in a technically satisfactory yield. The hexamethylenetetramine is prepared by oxidizing methane with air in the presence of ammonia gas. A mixture consisting of six volumes of methane, twelve volumes of oxygen, and four volumes of ammonia gas is passed through a constricted metal tube which is heated at the constriction. The tube is made of such a metal as copper, silver, nickel, steel, iron, or alloys of iron with tin, zinc, aluminum, or silicon or of iron coated with one of these metals. Contact material to act as a catalyst when non-catalytic tubes are used in the form of wire or sheets of silver, copper, tin, or alloys may be introduced in the tube. At atmospheric pressure a tube temperature... 

The amount of metallic nickel in catalysts was determined by a DTA method by Macak and Malecha (84). Nickel produced by the reduction of nickel oxide was reoxidized by oxygen and the AT of the oxidation reaction was determined by the apparatus. The maximum value of AT between the reactor for catalytic reaction and that with inert Si02 packing was proportional to the amount of nickel in the catalyst sample. Accuracy of the method was about 4%. 

Interestingly these complexes showed high activity without addition of alkyl aluminum compounds in the ionic liquid while they are almost inactive in toluene. These results are interpretable in terms of catalyst stabilization by the imidazolium-based ionic liquid. Reductive elimination of imidazolium is also possible as in toluene as in the ionic liquid but in the ionic liquid, a rapid reoxidation via addition of the solvent imidazolium cation seems possible and may prevent the formation of Ni deposits associated with catalyst deactivation. The carbene complex with R = n-Bu showed the highest activity with a dimer yield of 70.2% (TOF = 7020 h ). The preferred product of the nickel-catalyzed reaction is methylpentene. Additional phosphine ligand had no significant influence on the distribution of the products in this case. 

Figure I Reoxidation of secondary steam reforming catalyst bed. Profiles of a) nickel and (b) hydrogen concentrations as a function of time. 

In this example the temperature in an adiabatic bed during reoxidation is investigated by means of simulation. This requires the combination of a rate equation for the gas-solid reaction and of the model equations for the reactor. The first reaction to consider in the present case is the reaction between the oxygen of the gas phase and the hydrogen adsorbed on the catalyst. It is this hydrogen that is mainly responsible for the pyrophoric character of such a catalyst. The reaction between the oxygen and the adsorbed hydrogen will be considered to be infinitely fast. It causes a temperature rise that may initiate the oxidation of the nickel of the catalyst. 

Experience led to the introduction of catalysts based on nickel nitrate and oxalate, followed by lactate or formate as well as the original carbonates, all supported on infusorial earth, pmnice, or even charcoal to increase activity. Reduction procedures were foimd to be important in obtaining the highest catalyst activity. Reoxidation of nickel before use had to be avoided. Mixed nickel oxide and copper oxide reduced more easily than nickel oxide alone. ... 

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