Fuente: WIPO "circular economy" The present invention refers to a thermal process of two consecutive steps to control the formation of furfural in the valorization of lignocellulosic biomass rich in pentosans using alcoholic-acid eutectic mixtures. This process of two steps allows the control of secondary reactions and subsequent formation of inhibitor products in biomass pretreatment, improving the performance of further biomass transformations, such as the enzymatic and fermentative conversion of polysaccharides. The first step consists of the selective and high conversion of pentoses into pentosides in the presence of an alcoholic-acid eutectic mixture, in the absence of water, and at low temperature, inhibiting the occurrence of dehydration reactions of pentoses that lead to furfural formation. After separating the resulting liquid phase from the solid fraction enriched in cellulose, a second step relies on the conversion of formed pentosides into furfural using the same eutectic mixture in the presence of water and at elevated temperatures