Methyl glycolate (MG) is a key intermediate for synthesising various high-value products, playing a particularly important role in producing biodegradable polyglycolic acid. The selective hydrogenation of dimethyl oxalate (DMO), derived from coal-based syngas, to MG has attracted considerable attention due to its high atom economy and environmental friendliness. However, MG is prone to overhydrogenation to ethylene glycol (EG), a thermodynamically more favourable process. Silver-based catalysts offer excellent MG selectivity but suffer from high cost and sintering. Copper-based catalysts are low-cost but generally show low MG selectivity due to strong C–O hydrogenolysis and are prone to sintering at elevated temperatures above 240 °C. Transition metal phosphides, particularly Ni₃P, have emerged as a promising alternative due to their noble-metal-like electronic structure and thermal stability.
In a study published in ENG. Chem. Eng., researchers at the China University of Mining and Technology synthesised three spherical silica (SiO₂) carriers with similar particle diameters but different internal structures to load Ni₃P for DMO hydrogenation. To their surprise, despite their similar particle diameters and spherical morphologies, the three catalysts exhibited dramatically different activities. The catalyst with the largest specific surface area (304 m²·g⁻¹) showed the poorest performance, achieving only 55.4 % DMO conversion at 220 °C. The lowest-surface-area catalyst (46 m²·g⁻¹), which suffered from severe Ni₃P agglomeration due to insufficient Si–OH anchoring, still achieved 75.4 % conversion owing to its mesoporous structure. In contrast, the Ni₃P/SiO₂-I catalyst, despite its low surface area (54 m²·g⁻¹), achieved 92.2 % DMO conversion and 94.8 % MG selectivity at 220 °C, benefiting from abundant Si–OH groups and large mesopores (22.9 nm).
Through systematic characterisation, the team identified the density of surface Si–OH groups and pore size as the two carrier properties responsible for the observed differences. The Ni₃P/SiO₂-I catalyst, with abundant Si–OH groups and larger mesopores, achieved optimal regulation of the active component. The 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 enhanced DMO accessibility and timely MG diffusion. This combination promoted activity and maintained high MG selectivity by minimising overhydrogenation. In contrast, the micro-mesoporous structure of the high-surface-area Ni₃P/SiO₂-II compromised the formation of phase-pure Ni₃P and limited the effective utilisation of active sites. The Ni₃P/SiO₂-III catalyst, lacking sufficient Si–OH anchoring, suffered from severe Ni₃P agglomeration and formed non-target phases including Ni₁₂P₅ and Ni₂P.
The optimised Ni₃P/SiO₂-I catalyst also demonstrated excellent stability. Over 250 hours of continuous operation at 240 °C, DMO conversion remained stable at approximately 96 % and MG selectivity at roughly 92 %, with no obvious deactivation. The catalyst exhibited superior performance at both 220 and 240 °C with DMO conversion above 90 %. Although DMO conversion varied with changes in temperature, weight hourly space velocity, and H₂/DMO molar ratio, MG selectivity consistently exceeded 90 % across all conditions tested, attributed to the larger mesopores facilitating effective diffusion of MG away from the active sites.
This work reveals that beyond specific surface area, abundant Si–OH groups combined with large mesopores are key factors for Ni₃P catalytic performance in DMO hydrogenation. The findings provide clear design guidelines for developing more efficient and cost-effective catalysts, potentially lowering the production cost of biodegradable plastics and supporting the transition away from fossil-based materials.
DOI
10.1007/s11705-026-2702-6