Recycle and Extraction: Cornerstones for an Efficient Conversion of Cellulose into 5‑Hydroxymethylfurfural in Ionic Liquids

Hydrolysis and subsequent degradation of microcrystalline cellulose in five ionic liquids (ILs) using metal salts and/or Brønsted acids as catalysts allowed for the direct access to 5-hydroxymethylfurfural (HMF), an important renewable biofuel precursor and a useful building block. For each catalytic system, several reaction parameters (temperature, reaction time, catalyst, and cellulose loading) have been selectively changed. Four systems ([BMIM]­Cl-CrCl3, [BMIM]­Cl-FeCl3, [BMIM]­Cl-[MIMC4SO3H]­[HSO4] and the not yet investigated [BMIM]­Cl-TiOSO4) were found to be effective for cellulose degradation into HMF. The extraction of HMF from the reaction media represents however the weak point of all these processes being able to affect negatively both HMF recovery and IL recyclability. The critical step which causes the drastic decrease in HMF yield starting from the first recycle has been clearly identified. Furthermore, the possibility to use TiOSO4 as a sustainable and robust catalyst for the conversion of saccharides (or polysaccharides) in HMF has been shown. The present study could open new perspectives for the one-pot synthesis of HMF starting from cellulose and/or other sugars.