Reduced mixed Ni-Co-Al metal oxide catalysts with rich oxygen vacancy derived from layered double hydrotalcite for selective hydrogenation of furfuryl alcohol to 1,5-Pentanediol

Abstract

The C2-O cleavage of furanic ring is the crucial step in selective hydrogenation of furfuryl alcohol (FOL) to 1,5-pentanediol (1,5-PDO). In this study, reduced mixed Ni-Co-Al metal oxide catalysts with rich oxygen vacancy (O_v) and different Co/Ni molar ratios were prepared through intercalation modification of Co-based hydrotalcite by ammonium citrate (CA), followed by calcination and reduction. The catalytic performance exhibited that a quantitative conversion of FOL with 44.4 % yield and 8.2 mmol_1,5-PDO·g_cat^-1·h⁻¹ productivity of 1,5-PDO were achieved by using Co₂Ni₁Al₁O_x-CA(0.1) (molar ratio of Co:Ni = 2:1; molar concentration ratio of CA:Na₂CO₃ = 0.1) under optimal conditions. The stability test showed that Co₂Ni₁Al₁O_x-CA(0.1) consistently rendered above 40 % yield of 1,5-PDO in seven consecutive cycles. Catalyst characterizations were carried out using a series of techniques including XPS, EPR, O₂-TPD, etc. The results demonstrate that the addition of CA effectively altered the surface molar ratios of Co²⁺/(Co²⁺+Co³⁺), thereby regulating the O_v content of the obtained catalysts. The CoO-O_v sites in the catalyst might enhance the adsorption of FOL by η¹-(O)-alcoholic model, which weakened C2-O bond on the furanic ring of FOL. Besides, the H₂-TPD anslysis confirmed that the enhanced spillover of hydrogen from Ni⁰ onto CoO-O_v site, thereby promoting the cleavage of the C2-O bond in FOL and subsequent hydrogenation of enol intermediates. In addition, the DFT calculations imply that FOL adsorption on CoO-O_v site by η¹-(O)-alcoholic model was significantly favorable than that on pristine CoO sites (−1.68 eV versus −1.55 eV). Consequently, this study has substantiated the crucial role played by CoO-O_v in the reaction pathway leading to 1,5-PDO formation via FOL, proposing a viable scheme for designing catalysts based on transition metals and elucidating their underlying reaction mechanism.