Oxypropylation of lignocellulosic materials: A systematic review of processes, catalysts, and sustainable applications

Oxypropylation is a promising chemical modification technique that can add value to biomass and lignocellulosic waste, offering a sustainable alternative to petrochemical polymers. This systematic review provides an in-depth analysis of those modifications, covering from the production of reactive polyols for the synthesis of polyurethanes (PUs) to the obtaining of self-reinforced composites. Available data show that a large number of publications explore not only different sources of biomass but also a range of catalyst and isocyanate combinations. In the first case, commonly used catalysts are potassium hydroxide (KOH) and sodium hydroxide (NaOH), while the most used isocyanates are methylene diphenyl diisocyanate (MDI) and toluene diisocyanate (TDI). Furthermore, it also becomes clear that thermal and mechanical behaviour are the most investigated properties in oxypropylation-derived materials. These trends highlight the most targeted applications of these modifications, with rigid PU foams and composites being the most common resulting materials. A total of 65 articles published between 1967 and 2024 were analyzed. It could be observed that since 2012, there has been a significant increase in research on lignin modification aimed at improving its properties, particularly for PU applications. However, there are still gaps in oxypropylation studies, especially regarding the use of different catalysts, including other alkali metals. Additionally, the biodegradability of oxypropylated materials remains underexplored.