The role of the active phase of Raney-type Ni catalysts in the selective hydrogenation of D-glucose to D-sorbitol

Bulk and surface properties of homemade Raney-type Ni catalysts and their Ni-At alloy precursors have been studied and compared with commercial samples, both promoted and unpromoted. The two starting alloys had a different Ni2Al3 concentration. resulting in Raney-type Ni catalysts with different Ni-Al composition. All catalysts have been screened in the hydrogenation of an aqueous solution of D-glucose (10 wt.%) using a three-phase slurry reactor at 4.0 MPa and 393 K. The promoters are segregated at the surface of the catalysts and have a beneficial effect on the reaction rate, essentially due to an increased surface area and stability of the active phase while an enhanced interaction of D-glucose with Ni seems to play a secondary role. The Raney-type Ni catalysts lose Ni and Al at the applied reaction conditions. Mo does not leach at all. Fe leaches severely from the catalyst surface, but about 30% of the Fe at the surface is present as inactive, bulk iron. The major cause of deactivation of the Raney-type Ni catalysts is the presence of D-gluconic acid formed during the reaction. The effect of D-gluconic acid is two-fold: firstly it blocks the Ni sites making them inactive in D-glucose processing. The poisoned catalytic sites can be recovered again by a regeneration treatment in hydrogen at 393 K. Secondly, the formation of D-gluconic acid leads to a severe loss of Ni, which makes product purification necessary. (C) 2003 Published by Elsevier B.V.