Hydrothermal conversion of high-concentrated glycerol to lactic acid catalyzed by bimetallic CuAux (x = 0.01–0.04) nanoparticles and their reaction kinetics

Abstract

Catalytic hydrothermal conversion of highly concentrated glycerol to lactic acid was investigated over bimetallic CuAu_x (x = 0.01–0.04) nanoparticle catalysts. The bimetallic CuAu_x nanoparticles were prepared by the wetness chemical reduction method and characterized by XRD, TEM, HRTEM, XPS, and atomic absorption spectrophotometry techniques. Metallic Cu and Au nanoparticles coalesced to form secondary bimetallic nanoparticles with an alloy trend. The bimetallic CuAu_x nanoparticles exhibited higher catalytic activity for the conversion of highly concentrated glycerol (2−3 mol L⁻¹) to lactic acid than both sole monometallic Cu and Au nanoparticles, probably due to the alloying tendency between the metallic Cu and Au nanoparticles. When the reaction was carried out with the initial glycerol and NaOH concentrations of 3.0 and 3.3 mol L⁻¹ at 200 °C for 2 h, the yields of lactic acid over the bimetallic CuAu₂, CuAu₃, and CuAu₄ catalysts were above 83% and the formation rate of lactic acid was more than 0.17 mol g_cat⁻¹ h⁻¹. The carbon balance values ranged from 89.2% to 92.9%. The reaction activation energies for glycerol conversion over the bimetallic CuAu₁, CuAu₂, CuAu₃, and CuAu₄ catalysts were 64.0, 53.4, 46.8, and 36.9 kJ mol⁻¹, respectively. The hydrothermal conversion of high-concentrated glycerol to lactic acid catalyzed by the bimetallic CuAu_x nanoparticles is an alternative to the conventional fermentation process starting from carbohydrate.