Combined effects of Si-OH content and pore size of SiO₂ carriers on Ni₃P/SiO₂ catalysts for selective dimethyl oxalate hydrogenation to methyl glycolate

Abstract

The selective hydrogenation of dimethyl oxalate (DMO) to methyl glycolate (MG) is critical for biodegradable polyglycolic acid production, but MG is prone to overhydrogenation to ethylene glycol. Ni₃P/SiO₂ catalysts show promise in selective hydrogenation due to their unique electronic structure and thermal stability. The conventional strategies to enhance the performance of these catalysts rely on maximizing the carrier specific surface area to expose more active sites. In this study, we synthesized three spherical SiO₂ carriers with similar particle diameters but different internal structures to load Ni₃P. Unexpectedly, among the three catalysts, the highest-surface-area Ni₃P/SiO₂-II catalyst exhibited the best Ni₃P dispersion yet the poorest DMO conversion (only 55.4%), as its micro-mesoporous structure compromised the formation of phase-pure Ni₃P and limited the effective utilization of active sites. The lowest-surface-area Ni₃P/SiO₂-III, which lacks sufficient Si-OH anchoring, suffered from severe Ni₃P agglomeration but still achieved 75.4% DMO conversion owing to its mesoporous structure. In contrast, Ni₃P/SiO₂-I, despite its low surface area, achieved excellent performance with 92.2% DMO conversion and 94.8% MG selectivity at 220 °C, benefiting from abundant Si-OH groups and large mesopores. Systematic characterization revealed that the density of surface Si-OH groups and the pore size were the two carrier properties responsible for the observed differences in catalytic activity. Further analysis showed that abundant Si-OH groups suppressed Ni₃P agglomeration via chemical anchoring, while large mesopores enabled sufficient Ni-P precursor contact to ensure pure Ni₃P formation and also enhance DMO accessibility and timely MG diffusion. This combination promoted activity and maintained high MG selectivity by minimizing overhydrogenation. Thus, besides specific surface area, abundant Si-OH groups combined with large mesopores are key factors for Ni₃P catalytic performance in DMO hydrogenation.

Graphical abstract

Keywords

dimethyl oxalate / methyl glycolate / Ni₃P catalyst / silica carrier / hydrogenation

References

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