A review of bio-refining process intensification in catalytic conversion reactions, separations and purifications of hydroxymethylfurfural (HMF) and furfural

5-hydroxymethylfurfural (5-HMF) and furfural (FUR) have been recognized as valuable biomass-derived platform chemicals and bridge between biomass raw materials and the biorefinery industry, utilizing as a building blocks for the production of many chemicals and fuels. In this review, we aim to discuss the utilization of process intensification (PI) techniques within the biorefining industry, focusing on 5-HMF and FUR production and sequential purifications. These approaches such as reactive extraction in biphasic systems (in microreactor), reactive adsorption, reactive distillation (with stripping) and pervaporation assisted reaction are considered as the future of biorefining and have been surveyed in this paper. The reactive extraction in a tubular slug-flow microreactor and a stirred reactor-extractor-settler configuration, as the most simple, cheap and effective approach, are discussed. In general, the highest product yields were observed in an aqueous system catalyzed with mineral acids, while the purification and recycling of acids is also laborious and very costly which exacerbates its application. The combination of the solid-acid catalysts (metal oxides, silica, alumina, metal phosphates, zeolites, heteropolyacids or their mixture) in water/MIBK and water/THF biphasic solvent systems with adding of NaCl (which improved both extraction power and product selectivity) is the most promising approach. The best reaction performance was demonstrated in the microreactor but at the same time, the latter is sensitive to plugging from humins formation with long time-on-stream. Finally, for the large scale of FUR production, the pervaporation assisted continuous membrane reactor showed one of the highest potential.