Ionic Liquids: Synthetic Methodology and ApplicationsA Chronological and Sustainability-Focused Review (2019-2026)

Abstract

Ionic liquids (ILs) - salts that melt below 100 degC - have been recognised as highly versatile and customisable materials, exhibiting negligible vapour pressure, broad electrochemical windows, and outstanding solvation characteristics. This review consolidates advancements in synthetic methodology and diverse applications reported between 2019 and 2026, organising the synthetic literature chronologically to make the pace and direction of the field explicit, and emphasising the transition toward fourth-generation bio-based, biodegradable, and multifunctional ionic liquids sourced from renewable precursors.

Traditional synthesis methods - quaternization, neutralisation, and metathesis - continue to serve as essential techniques, while green methodologies including microwave- and ultrasound-assisted processes, mechanochemical (ball-milling) techniques, one-pot reactions, and bio-derived approaches offer efficient, high-yield, halide-free alternatives. Task-specific functionalisation enables precise applications across catalysis (heterocycle synthesis, biomass valorisation), energy storage (high-voltage battery and supercapacitor electrolytes, CO2 electroreduction), environmental separations (CO2 capture exceeding 1 mol/mol, metal and rare-earth-element extraction), biomedicine (drug solubilisation, transdermal delivery, antimicrobial agents), and industrial processes (alkylation, lubricants, cellulose fibres).

Persistent challenges - high production cost, viscosity-limited mass transfer, variable toxicity, and regulatory hurdles - are weighed against a projected market growth to USD 110-136 million by 2032-2034 (CAGR 8-13.5%). Future progress is expected to depend on AI-driven molecular design, hybrid solvent systems, and circular-economy principles to realise the potential of ILs in sustainable, low-carbon technologies.