Nickel Catalysts and Catalytic Processes

Although the hydrogenation reaction can be catalyzed by many metals, nickel is most commonly used for three important practical reasons low cost, good catalytic activity, and metal inertness. The main problems associated with the use of Ni catalysts are the low selectivity and the high activity for isomerization reactions. 

The activity is determined by the Ni surface and its accessibility, which also affects the selectivity of the process. Thus the active phase is usually dispersed on a solid support, which also acts as a filter aid and even as promoter. The 

Kieselguhr is the most used support. Improvements in supports have been made synthetic silica or silicates have shown better catalyst reproducibility from batch to batch, with an increase in Ni surface area and a decrease of mean particle size. 

With this type of catalyst, the hydrogenation is usually carried out in the ranges 110-190°C, under 1-5 bar H2 with 0.01-0.15% Ni catalyst (w/w). The hydrogenation of fats is somewhat special due to the need to work in all three phases (gas, liquid, and solid, with corresponding mass and heat transfer problems), and since the natural feedstocks used show significant variations in composition. For these reasons batch reactors are still preferred because of their simplicity, lower cost, and since they have the flexibility to be adapted to different feedstocks or different end products. 

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