A Study On Non-stoichiometric Copper-and Nickel-Based Perovskite Catalysts For Selective Hydrogenation Of Levulinic Acid

The family of perovskite oxides is an important functional material with the formula of ABO3 where A site could be rare earth or alkaline earth elements and B site could be transition metal elements.Typically,perovskite oxides have high compositional flexibility,which can involve?90%of the elements in the periodic table.Suitable amount of substitution or doping into A or B site still maintains its structural formula.Basically,the catalytically active B-site metals in the perovskite crystal lattice are closely coordinated with oxygen ions with high oxidative state and thermal stability,which enable perovskite as promising candidate for high temperature gas-liquid-solid reaction.Recent study has shown that after reducing gas treatment,the B-site dopants in non-stoichiometric perovskite could segregate onto the perovskite surface as metallic nanoparticle with "socketed" state and uniform dispersion.Such metal/oxide heterostructures have been applied as novel catalysts for diverse heterocatalysis processes.Usually,metal copper and nickel are two active elements which are commonly used in the field of catalytic hydrogenation.However,copper-and nickel-based catalysts prepared by traditional impregnation method suffer from several drawbacks,such as easy loss,aggregation,sintering of active metals,and poor stability.In comparison,the non-stoichiometric copper-and nickel-based perovskite catalysts have better selective hydrogenation performance after in-situ precipitation of active species(Cu and Ni).In this paper,the non-stoichiometric copper-based and nickel-based perovskite catalysts were synthesized by sol-gel method and reduction.The selective hydrogenation of levulinic acid to y-valerolactone was chosen as the target reaction for such series of catalysts.The catalysts with surface-dispersed active nanoparticles were analyzed by various structural characterization methods to obtain the basic structural-performance relationship.For the copper-based perovskite,the catalytic reaction was carried out for 2.0 h at a reaction temperature of 473 K under a pressure of 2.0 MPa H2 to completely convert levulinic acid into y-valerolactone.However,the hydrogenation performance is significantly decreased after three cycles.Detailed study has shown that the perovskite structure was not maintained,and copper aggregation occurred on the surface,which may be due to the low lattice stability of Cu ions with its low chemical valence.Further modification of B-site by doping other metals cannot improve the structure stability of the copper-based perovskite,as confirmed in the experimental results.Nickel-based perovskite was further prepared and investigated to obtain catalysts with stable structure and catalytic performance.The results showed that,levulinic acid was almost completely converted to y-valerolactone at a reaction temperature of 473 K at 2.0 MPa H2 for 2.0 h with a selectivity of 99.8%.After 5 cycles of hydrogenation reaction,the catalyst still reached over 90%levulinic acid conversion.For ?-valerolactone selectivity,it decreased from 99.8%to 82%after 4 cycles,and the selectivity decreased to 62%at the 5th cycle.After 5 cycles,although the perovskite phase structure of the catalyst surface is weakened,there is no obvious metal nickel species on the surface.Confirmed by the results of SEM,XRD and H2-TPR,the active Ni0 nanoparticles reduced on the surface of perovskite are the reaction site for the selective hydrogenation of levulinic acid,and the stable perovskite phase structure can prevent sintering,maintain the uniform dispersion of nanoparticles and enhance the stability of the catalyst.