This invention relates to a novel molten salt system in chemical transformation of saccharides and a method as well as an apparatus for multi carbon production by the molten salt system. It is found that an eutectic molten salt composition is advantageous for multi-carbon productions through chemical transformation under mild conditions. This invention further provides a method and an apparatus for preparing 5-hydroxymethylfurfural (HMF) and HMF-derived chemicals from saccharides by the said molten salt system.