Structure sensitivity of Cu supported on manganese oxide catalysts in levulinic acid hydrogenation

Different synthesis methods were used to prepare a series of size-controlled copper nanoparticles supported on manganese oxide octahedral molecular sieve (OMS-2) catalysts. All Cu/OMS-2 catalysts, with average Cu nanoparticle sizes prepared in the range of 2-22 nm, were thoroughly characterised using X-ray diffraction (XRD), N-2 sorption, H-2 temperature programmed reduction (TPR), transmission electron microscopy (TEM), and ICP-OES elemental analyses. The catalytic activity of the size-controlled Cu/OMS-2 catalysts was investigated in liquid phase hydrogenation of levulinic acid as a model reaction to evaluate the nanoparticle size dependance and structure-activity relationship. The catalytic activity studies showed that the catalyst performance depends greatly on the catalyst preparation methodology and Cu nanoparticle size. Complete conversion of levulinic acid with a high gamma-valerolactone yield, >99%, was observed using Cu/OMS-2 catalysts prepared by the precipitation-deposition (Cu nanoparticle size 2-3 nm) method. In comparison to wet-impregnated catalysts (Cu particle size 20-22 nm), the improved performance of precipitation-deposition catalysts was mainly attributed to the well-distributed, smaller Cu nanoparticles. The influence of Cu nanoparticle size is correlated with the turnover frequency (TOF, h(-1)) for levulinic acid conversion, indicating the structure sensitivity of the levulinic acid hydrogenation reaction.