Nickel-silica, hydrogenation

Lok, C M. (1992) Nickel/Silica Hydrogenation Catalysts and the Preparation and Their Use in Hydrogenating Unsaturated Patty Acids. U.S. patent 5112792. 

As in the nickel/silica case, the rate of hydrogen reduction of nickel oxide... 

Fig. 2. (a) Magnetization-volume isotherms for the chemisorption of hydrogen and of benzene on kieselguhr-supported nickel at 150° C (16). (b) Average number of bonds formed by benzene adsorbed on nickel-silica as a function of temperature (17). From J. Amer. Chem. Soc. 79, 4637 (1957) 83, 1033 (1961). Copyright by the American Chemical Society. Reprinted by permission of copyright owner. 

Specific Activities of Various Nickel-Silicas for Benzene Hydrogenation (Ah) and Exchange (A )... 

Adsorption of hex-l-ene, a mixture of cis- and frans-hex-2-ene, and c/s-hex-3-ene on nickel—silica results in identical infrared spectra [83]. Addition of hydrogen results in an intensification of the spectrum suggesting that the initial spectrum results from dissociatively adsorbed species, a conclusion substantiated by the observation that the gas in equilibrium with the surface during the initial adsorption contains isomerised hexenes. Evacuation of the hydrogen causes a decrease in intensity and the reappearance of the initial spectrum. 

In the above diagram, H-D refers to hydrogenation-dehydrogenation centers and A to acidic centers on the catalyst. The reaction sequence involves successive ring contraction and expansion steps, similar to the mechanism proposed by Pines and Shaw (P4) to account for transfer of tagged carbon from the side chain to the ring when ethylcyclohexane was contacted with a nickel-silica-alumina catalyst. 

P4-22g Alkylated cyclohexanols are important intermediates in the fragrance and perfume industry [Jnd. Eng. Chem. Res., 28, 693 (1989)]. Recent work has focused on gas-phase catalyzed hydrogenation of o-cresol to 2-methylcyclo-hexanone, which is then hydrogenated to 2-methylcyclohexanol, In this problem we focus on only the first step in the reaction (Figure P4-22). The reaction on a nickel-silica catalyst was found to be zero-order in o-cresol and first-order in hydrogen with a specific reaction rate at 170°C of 1.74 mol of o-cresol/(kg cat - min - atm). The reaction mixture enters the packed-bed reactor at a total pressure of 5 atm. The molar feed consists of 67% Hj and 33% o-creso at a total molar rate of 40 mol/min. 

Direct evidence for the participation of any of the foregoing species in hydrogenation reactions is scant. Structure (A) was believed to be form-ed when ethylene is admitted to hydrogen-covered nickel-silica (8). Evidence for Structure (B) is provided by the observation that the surface potential of ethylene on nickel film is -f 0.83 volts (24) and not negative as would be expected for Structure (A) on electronegativity considerations. Until such time as experimental methods for the direct observation of adsorbed species under reaction conditions are perfected, we must discuss possible reaction mechanisms in the most general way. 

The reaction of 1-hexene with deuterium over nickel-silica at 105° yields hexanes distributed as shown in Table VI (49) no observations on isomerization or exchange were made. The hexanes become progressively more deuterated as the D /hexene ratio is raised. Isomerization of n-hexenes has been observed during their hydrogenation over Raney nickel (50). 

This effect, however, is discounted by Smith (34) who suggests that initial hydrogen pressure, amount of catalyst and type of solvent may have had some influence on reaction time. In some other work by Adkins (35) there was little significant difference between the reduction of 2- and 3-methylpyridines. Similarly, hydrogenation in the presence of a nickel silica gel catalyst shows more rapid conversion for pyridine (5 hr) than for 2-methylpyridine (7 hr) (36)... 

Fig. 1. Relative magnetization vs. absolute temperature for (1) coprecipitated nickel-silica containing 34% Ni, (2) Universal Oil Products Co. nickel hydrogenation catalyst containing 52% Ni, and (3) the same sintered for 6 hrs. at 650°. All reductions were in flowing hydrogen for 12 hrs. at 350°.

Fig. 4. Magnetization vs. temperature for a partially sintered 40% nickel-silica coprecipitate before and after adsorption of hydrogen at room temperature. This permits a comparison of electrons taken in per atom of nickel with hydrogen atoms adsorbed per atom of nickel.

Fig. 5. Effect of hydrogen adsorbed at room temperature on a 26% nickel-silica coprecipitate showing diminishing influence of hydrogen on smallest particles of nickel (i.e., those observed at low temperatures only).

Fig. 8. Automatic recording of magnetization changes occurring during adsorption and desorption of hydrogen on 34% nickel-silica, at 27°.

B. Nickel-Silica Catalyst It was prepared by impregnating wet silica gel with nickel nitrate solution, drying the mass, and decomposing the nitrate to the oxide at 350-400°. It was subsequently reduced in a stream of hydrogen at 300-350°. 

Cycloheptane and cyclooetane at 200-250 C when hydrogenated over nickel undergo changes to methyl and dimethyl derivatives of Cyclopehtane and cyclohexane. Thus, methylcyclohexane in the presence of a nickel-silica-alumina catalyst at 290-370 C and in the presence of hydrogen gives a mixture of 1,1-, 1,2-, and 1,3-dimethyIcyclopentanes and some ethyl-cyclopentane. ... 

Figure4.15 Carbon monoxidecontent in the hydrogen product of methane cracking over nickel/silica and nickel/zeolite Y weight hourly space velocity 20 L (h gcat) temperature 550 C [265].

A particular issue is the deactivation of methanation catalysts by carbon formation. Kuijpers et al. [345] observed significant carbon formation over a nickel/kieselgur catalyst containing 54wt.% nickel when exposed to a mixture of 10 vol.% carbon monoxide, 15 wt.% hydrogen, with a balance of nitrogen at 0.6 bar pressure and a 250 °C reaction temperature. Carbon filaments were found, which contributed to 10 wt.% of the catalyst mass at the inlet of the fixed bed. A nickel/silica catalyst showed practically no coke formation for 1000 h duration under the same conditions. 

The catalysts used in the process are essentially nickel metal dispersed on a support material consisting of various oxide mixtures such as alumina, silica, lime, magnesia, and compounds such as calcium aluminate cements. When the catalyst is made, the nickel is present as nickel oxide which is reduced in the plant converter with hydrogen, usually the 3 1 H2 N2 synthesis gas ... 

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