GLUCOSE HYDROGENATION ON PROMOTED RANEY-NICKEL CATALYSTS

Glucose hydrogenation has been studied in a well stirred, high-pressure batch reactor on promoted Raney-nickel catalysts. Mo-, Cr-, and Fe-promoted catalysts were prepared by soda attack on Ni40-xAl60Mx alloys. Sn-promoted catalysts were obtained by controlled surface reaction of Sn(Bu)4 on the hydrogen-covered surface of a Raney-nickel obtained from a Ni2Al3 alloy. The loading of tin is stoichiometric and its distribution on the nickel surface is very homogeneous down to nanometer scale. For an optimum promoter concentration the catalysts are up to seven times more active than unpromoted ones. A good distribution of the promoter in the catalyst grain is required to obtain the best rate enhancement; in the case of molybdenum this is obtained by annealing the alloys. The promoters in a low-valent state on the nickel surface act as Lewis adsorption sites for the oxygen atom of the carbonyl group which is then polarized and thus more easily hydrogenated via a nucleophilic attack on the carbon atom by hydride ions. The activities of Mo- and Cr-promoted catalysts decrease slightly after several recyclings in successive hydrogenation experiments. This is mostly due to surface poisoning by cracking products formed in side reactions. Fe- and Sn-promoted Raney-nickel catalysts deactivate very rapidly because Fe and Sn are leached away from the surface. Iron is washed to the liquid phase whereas tin remains in the Raney-nickel microporeS. (C) 1994 Academic Press, Inc.