Solid raney-type Ni foam catalysts for xylitol production: Continuous and batch operation

Solid activated metal foam catalysts represent a promising alternative for the continuous production of valuable sugar alcohols. Traditionally, sugar alcohols are produced industrially in batch mode using finely dispersed Raney-type nickel catalysts. In this study, novel solid foam Raney-type Ni catalysts (activated metal foam catalysts) were used for the hydrogenation of xylose to xylitol in both batch and continuous operation. Two types of catalysts were investigated: Raney-type Ni foam (Metalyst® MC 911 by Evonik Operations GmbH, Ev-F-Ni) and Raney-type Ni foam promoted with molybdenum (Metalyst® MC 981 by Evonik Operations GmbH, Ev-F-NiMo). Catalyst deactivation was primarily attributed to the accumulation of strongly adsorbed organic species on the active sites and to Ni and Al leaching, which reduced the availability of catalytically active sites. Ev-F-NiMo demonstrated a superior stability and activity compared to Ev-F-Ni, attributed to electronic interactions between Mo and Ni, which stabilize Ni in a lower oxidation state and reduce metal leaching under reaction conditions. In continuous operation, the catalysts exhibited reduced deactivation, likely due to enhanced desorption of poisons under the continuous flow of fresh feed. Although the exposure to xylonic acid, a potential poison forming on the catalyst surface, temporarily reduced the xylitol yield in continuous mode, the Ev-F-NiMo catalyst demonstrated good resilience, recovering its activity after the removal of the poisoning species. These results highlight the very attractive technical solution for the continuous production of sugar alcohols from sugar monomers, utilizing promoted Raney-type Ni catalyst that is highly active, selective and cost-effective.