Direct solvent-free selective hydrogenation of levulinic acid to valeric acid over multi-metal [Ni_xCo_yMn_zAl_w]-doped mesoporous silica catalysts

Valeric acid (VA) and valeric esters (VEs) are among the essential platform chemicals derived from bio-levulinic acid (LA). However, synthesizing VA from LA is still challenging due to solvent consumption, precious metals utilization, and catalyst deactivation. Thus, this study developed an efficient direct solvent-free selective hydrogenation reaction of LA to VA using non-noble metal-based catalysts. Ni_xCo_yMn_zAl_w-doped mesoporous silica (NMCAS) catalysts were synthesized using a facile one-pot procedure, with 100% LA conversion and 96.3% VA yield obtained at 200 °C and 3 MPa (H₂) over the NMCAS catalyst. Ni-based catalysts provided 100% LA conversion, whereas the Mn-Co-based catalyst exhibited a 79.8% LA conversion. The increase in the selectivity for VA observed with the doping of Mn and Al into the catalysts can be explained by the generation of Brønsted and Lewis acid sites, which facilitated the ring-opening of GVL to form VA. The effects of reaction time, temperature, pressure, and solvent were considered. At elevated temperatures and pressures, the formation of 1,4-PDO, resulting from the C[sbnd]O bond breakage of O-C–O in the GVL ring-opening and the subsequent dehydration-cyclization to 2-methyltetrahydrofuran (MTHF), including the over-hydrogenation of VA to pentanol, was detected. An increase in the partial pressure of hydrogen enhanced the hydrogenation activity, thus producing 2-MTHF and pentanols. The NMCAS catalyst has also been applied with various solvents to study the reaction pathway and produce VEs. This work is the primary example of the direct solvent-free, selective, and high-yield production of VA from LA over non-noble heterogeneous catalysts.