Optimizing lactic acid production for agricultural wastes revalorization via sugars hydrothermal conversion with zirconia-based catalysts: response surface methodology

Abstract

The hydrothermal conversion of a complex sugars solution comprising glucose, arabinose and xylose with zirconia-based catalysts to produce lactic acid (LA) was studied. Catalysts were synthesized by a simple template-assisted sol-gel method. Alternative template removal procedures, such as refluxing ethanol extraction, were compared to the conventional direct calcination. The catalyst obtained after ethanol extraction (Z-Ex) reported the highest specific surface area (323 m2 g-1), total pore volume (0.21 cm3 g-1) and density of acid sites (1.111 µmol m-2), with a significant contribution of strong acid sites. On the other hand, template removal by thermal treatment transformed the hydrous zirconia into crystalline tetragonal phase, reducing the specific surface area and removing most of the acid sites. Catalytic tests were carried out in a stirred batch reactor, resulting Z-Ex the most selective towards LA production. A four factors experimental design was performed for Z-Ex catalyst by varying temperature, reaction time, initial pressure and catalyst/reactants ratio. Results were analyzed using the response surface methodology (RSM) to maximize the LA production. A molar yield of 61.2 % towards LA and complete sugars conversion were reached at optimized operating conditions: 189 °C, 208 min, 0.5 g catalyst g-1 reactants and 10 bar N2. Additionally, fitting models for main by-products molar yields (furans, glyceraldehyde, glycolic, formic, acetic and levulinic acids) were obtained with good accuracy prediction. All responses were jointly studied to understand the reaction mechanisms involved in generation of intermediaries and undesired degradation products.