Green pretreatments and separation challenges in lignocellulosic biomass valorization: A review

• Green pretreatment strategies for delignification of lignocellulosic biomass. • Fungal pretreatment stands out as environmentally friendly. • The downstream process remains one of the main bottlenecks. • Integrating pretreatment and separation processes is essential. Lignocellulosic biomass is a promising source for the production of fuels, materials, chemicals, and energy due to its low cost, wide global availability, and estimated annual generation of 200 billion tons. Although sectors such as biofuel production and the paper industry already use consolidated conversion routes, the advancement of integrated biorefineries still faces significant technological bottlenecks, especially in the pretreatment and downstream stages. In this context, the search for more sustainable alternatives becomes crucial, especially in sectors that are difficult to decarbonize, such as the chemical industry. Green pretreatments have been recognized for their ability to reduce energy consumption, minimize pollutant release, and decrease greenhouse gas emissions associated with biomass conversion. However, despite the large number of studies involving pretreatments and valorization routes, the separation and purification stages remain underexplored. These stages can account for 20 to 40% of the total production costs, constituting one of the primary bottlenecks to the viability of biorefineries. Given these limitations, this review provides an updated and comprehensive overview by integrating data from recent studies that employ machine learning, life cycle assessments, technoeconomic analysis, and multi-omics approaches to optimize the biological delignification process, with a special emphasis on advances in fungal pretreatment. Thus, the article discusses not only the progress in green pretreatments but also the persistent challenges in the downstream steps of compounds of interest for the valorization of lignocellulosic biomass.