Conversion of glycerol to lactic acid over Cu–Zn–Al and Cu–Cr catalysts in alkaline solution

Abstract

Abstract A Cu–Zn–Al catalyst (from Johnson Matthey) and a Cu–Cr catalyst (form Strem Chemicals) were used for catalyzing glycerol dehydrogenation to produce lactic acid (LA) in alkaline aqueous solution. These catalysts were pre-reduced with hydrogen before use. Cu–Zn–Al exhibited much better performances than Cu–Cr, because the former had much larger surface area and higher copper content than the latter. The effects of reaction temperature (between 100 and 175 °C) reaction time, alkaline and catalyst concentrations on glycerol conversion and product yields were studied for the Cu–Zn–Al catalyst. LA selectivity was essentially constant (∼85%) until 95% conversion, and reached 96% at 98.9% conversion (LA yield was 94.9%) and 175 °C, which was much better than that reported previously with a CuO/Al2O3 catalyst. The rate of glycerol disappearance exhibited first-order dependence on glycerol concentration, and the rate constant increased linearly with catalyst concentration, suggesting that glycerol dehydrogenation on copper surface was the rate-limiting step. The yields of two major by-products (1, 2-propanediol and glyceric acid) decreased when glycerol conversion was high, ascribed to the reversibility of the by-products formations and the decrease of 1, 2-propanediol/LA generating rate ratio with increasing alkaline concentration.