New technology for hydrogenation processes in the alimentary industry

This doctoral thesis explores novel catalytic technologies for the selective hydrogenation of biomass-derived sugars into sugar alcohols, molecules with high relevance for the alimentary industry. The work focuses on structured catalysts, particularly metallic solid foams, owing to their exceptional properties: high geometrical surface area, thin catalytic layers that suppress internal mass transfer resistance, and open structures that minimize pressure drop in packed-bed reactors. These features make solid foams promising alternatives to conventional catalyst particles and slurry systems, and suitable candidates for enabling the transition from semi-batch to continuous operation.
Three types of catalysts were investigated. First, ruthenium supported on mesoporous molecular sieves (MCM-41, SBA-15, and MCF) were synthesized as silicates and aluminosilicates and tested in xylose hydrogenation. The incorporation of aluminium improved ruthenium dispersion and activity, although stability was limited by metal leaching. Second, Raney-type solid nickel foams, with and without molybdenum promotion, were studied. While active and selective, the typical stability issues of nickel catalysts were mitigated by addition of molybdenum, which doubled the activity and enhanced resistance to deactivation. Third, Ru/C solid foams were synthesized via carbon coating of aluminium foams using poly(furfuryl alcohol) as precursor, with pore tailoring achieved through polyethylene glycol (8 kDa) addition. These catalysts exhibited excellent activity, selectivity, and reusability, making them suitable for both semi-batch and continuous operation.
Extensive kinetic experiments were performed in semi-batch reactors with monomeric sugars and mixtures (xylose, arabinose, galactose). Mechanistic models based on non-competitive and semi-competitive adsorption provided excellent agreement with experimental data. The competitiveness factor (α ≈ 0.74) of the semi-competitive adsorption model supported the predominance of sugar adsorption while leaving interstitial sites accessible for hydrogen, consistent with molecular size differences.
Continuous hydrogenation studies were conducted in a laboratory scale parallel screening reactor in trickle-bed regime. The effects of temperature, liquid flow rate, and concentration on conversion, selectivity, and space–time yield were quantified. Hydrodynamic measurements (residence-time distribution, liquid holdup) were coupled with kinetic modelling, demonstrating that gas–liquid and liquid–solid mass transfer resistances are of comparable magnitude under the low-interaction regime. At higher sugar concentrations and flow rates, liquid–solid hydrogen transfer emerged as the prevailing limitation.
Overall, this thesis demonstrates that structured foam catalysts overcome the internal diffusion and pressure drop limitations of conventional catalyst particles, while maintaining excellent activity and selectivity in sugar hydrogenation. The combined insights from material development, intrinsic kinetics, and transport modelling provide a robust framework for designing efficient three-phase catalytic systems. Future research should investigate reactor configurations and operating strategies that enhance wetting and interfacial mass transport, thereby unlocking the industrial potential of foam-based catalysts in biomass valorization.