Recent progress in the separation of 5-hydroxymethylfurfural and its derivatives from ionic liquids

Abstract

Fossil resources, such as coal, oil, and natural gas, provide approximately 86% of energy and 96% of organic chemicals globally. Recently, however, with the continuous reduction in fossil resource reserves and the gradual increase in fossil resource prices, identifying renewable resources that can replace fossil resources has become particularly critical. Currently, as a renewable resource, biomass is low-cost, with extensive sources and abundant reserves; therefore, it has attracted increasing attention for use in energy and resource applications. Among them, the use of biomass to produce energy and chemicals is a cutting-edge research field in both domestic and industrial sectors. The preparation of 5-hydroxymethylfurfural (5-HMF) has become a research frontier for the comprehensive utilization of biomass resources. 5-HMF and its derivatives are crucial platform compounds that may not only be used in producing liquid fuels, such as furans, alkanes, and acetyl propionate, but also in producing solvents, pharmaceutical intermediates, and polyester materials. Traditional methods of preparing 5-HMF and its derivatives mostly use water or organic solvents as the reaction solvents. Water is green and cost effective, rendering it an ideal reaction solvent. In contrast, 5-HMF is prone to further hydration in water to produce by-products, such as acetylpropionic acid and formic acid, thereby reducing the yield of 5-HMF. Although organic solvents (for example, dimethyl sulfoxide, N,N-dimethylacetamide, and N,N-dimethylformamide) can inhibit the production of by-products to a certain extent, their boiling points are often relatively high; this may hinder the separation and purification of 5-HMF and may also cause severe environmental pollution. Recently, ionic liquids have provided novel opportunities for the preparation of 5-HMF and its derivatives because of their excellent physicochemical properties, such as low volatility, low melting point, low toxicity, high thermal stability, strong adjustability, and solubility. Numerous investigations have been conducted regarding the application of ionic liquids as solvents or catalysts in producing 5-HMF and its derivatives. The use of ionic liquids has suppressed the occurrence of side reactions, and increased the selectivities and inversion rates of 5-HMF and its derivatives. Accordingly, the application of ionic liquids to improve the rates and yields of carbohydrate conversion to 5-HMF and its derivatives has received increasing attention. However, research regarding the separation of 5-HMF and its derivatives from ionic liquids and the recycling of the reaction media is limited, which is a major obstacle in the industrial preparation of 5-HMF and its derivatives in ionic liquids. Therefore, separating 5-HMF and its derivatives from ionic liquids has attracted widespread attention, and a few studies have been reported in this regard. To elucidate the current research status regarding the separation of 5-HMF and its derivatives from ionic liquids, this review comprehensively summarises and analyses the state-of-the-art achievements based on various methods of isolating 5-HMF and its derivatives. Critical parameters, such as the distribution coefficient, adsorption capacity, separation selectivity, and recycling efficiency, were used to evaluate the application and separation mechanisms of liquid-liquid extraction, direct precipitation, adsorption, nanofiltration, supercritical CO2 technology, indirect distillation, and sublimation. Finally, this review provides an overview of the main issues (such as screening and developing suitable separation technologies, design and development intelligent ionic liquids, the research on the application of molecular dynamics simulation and Al technology in the separation mechanisms, and the feasibility study of the technology and economy) that remain to be studied, with the aim of providing useful references for use in the selective, economical separation of 5-HMF and its derivatives during biomass refining processes.