Roadmap for enhanced CO2 transfer hydrogenation with bio-derived glycerol over Ni-zeolite catalysts: Influence of zeolite structure and Si/Al ratio

This works presents, for the first time, a detailed investigation into the impact of zeolite structure type (ZSM-5, BEA and Y) and Si/Al ratio (11.5–200) on the CO2 transfer hydrogenation with bio-glycerol over Ni-impregnated zeolites, at 225 °C, 10 bar and 1 M NaOH. ZSM-5 revealed as the best zeolite support owing to its higher reduced Ni availability, better hydrophobicity and suitable basic properties, compared to Y and BEA zeolites. However, while glycerol conversion is greatly influenced by the zeolite properties, CO2 hydrogenation to formic acid is primarily catalysed by NaOH under the tested conditions. Glycerol dehydrogenation to lactic acid was identified as the main glycerol pathway, but glycerol reforming and dehydration extent increase with the Ni/zeolites. Maximum lactic acid (5.8 %) and hydrogen yields (9.4 %) were obtained after 2 h for Ni/ZSM-5 with SiAl = 40, increasing to 12.2 and 17.2 % respectively after 24 h. This showed that good basicity and hydrophobic character are key for improved glycerol dehydrogenation. Moreover, while hydrophobicity increases glycerol dehydration, glycerol reforming is mainly affected by Ni particle size. Conversely, formic acid yield after 2 h was similar for all Ni/zeolites (1–1.4 %). Catalytic tests at different reaction times and recycling tests revealed the importance of pH variations to the structural integrity of the zeolites, and so stability, opening the door to the possibility that selectively desilicated zeolites might optimise catalytic activity for this process. Overall, the findings elucidate the structure–function relationships in zeolites that favour CO₂ transfer hydrogenation with glycerol, enabling targeted strategies for future catalyst enhancement.