External intensifications for ionic liquids-based biorefinery in highly viscous systems

Over the past two decades, ionic liquids (ILs) have become effective media for biomanufacturing, primarily enabling the dissolution and separation of lignocellulosic biomass (biomass hereafter), while also facilitating its conversion. However, their application is challenged by the high viscosity of ILs, worsened by dissolving biomass polymers, which limits mass and heat transfer. These limitations increase IL consumption, energy use, and processing times, hindering industrial scale-up and sustainability. To overcome this, researchers have developed strategies such as adding co-solvents and applying external energy fields. This review summarizes recent studies on these intensifications in IL-biomass systems, focusing on methods that enhance processing without chemically modifying ILs. We outline fundamental principles of IL-based biomass processing and the transfer limitations caused by viscosity, then analyze molecular mechanisms behind viscosity increase. The core discussion covers two main strategies: (1) co-solvent addition, which reduces viscosity by diluting IL and modifying IL-IL and IL-biomass interactions; and (2) external fields, including microwave irradiation that improves heat transfer via dipole relaxation and ion polarization, and ultrasound treatment that enhances mass transfer through cavitation-induced microjets. Finally, current challenges and prospects are critically assessed to guide more sustainable and energy-efficient IL-based biomass processing technologies.