Rational design of hydroxyapatite/graphite-supported bimetallic Cu–M (M = Cu, Fe, Co, Ni) catalysts for enhancing the partial hydrogenation of dimethyl oxalate to methyl glycolate

Abstract

In the sequential hydrogenation reaction of dimethyl oxalate (DMO → MG → EG → EtOH), the control of product distribution to methyl glycolate (MG) is challenging, therefore noble metal-promoted Cu-catalysts were basically used to achieve such a partial hydrogenation step. In this study, for the first time, we rationally designed noble metal-free hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂)/graphite (HAp/G) composite catalysts with novel architectures decorated with transition metal nanoparticles (Cu, FeCu, NiCu and CoCu), which were then utilized in the DMO hydrogenation reaction. The DMO catalytic hydrogenation results revealed that among the as-made catalysts, CoCu/HAp/G exhibited the highest MG selectivity of 99.3% at 220 °C. Notably, the calculated turnover frequency (TOF) and MG selectivity were found to increase with the increase of the experimentally measured work function (Φ_WF) values for the studied catalysts (following the order: FeCu/HAp/G < Cu/HAp/G < NiCu/HAp/G < CoCu/HAp/G). Moreover, Auger Cu-LMM analysis reveals that cobalt species have increased the concentration of active Cu⁰ that is crucial for the partial hydrogenation step, while Cu⁺ is important for maintaining the stability of MG selectivity with the increase of reaction temperatures. The rationale for the exceptional catalytic activity and stability of the produced catalysts are explained in terms of the improved reducibility of copper, unique textural properties with an extra-large pore diameter and the synergistic effect between Cu⁰/Cu⁺ and Co⁰ dual active sites.