Recent Progress Towards Transition Metal-Catalyzed Direct Conversion of Cellulose to 5-Hydroxymethylfurfural

5-Hydroxymethylfurfural (HMF) has been considered as a green platform chemical with a wide range of applications due to its rich chemistry and potential availability from renewable biomass resources. In recent years, considerable efforts have been made to transform carbohydrates into HMF using homogeneous and heterogeneous acid catalysis. In this critical review, the direct conversion of cellulose into HMF catalyzed by transition metals is described because of the present important utility of such metals. Since solid acids are deemed less noxious than traditional liquid acids, significant attention is given to hydrolysis, isomerization, and dehydration reactions successively carried out in a one-pot mode from the polysaccharide using heterogeneous catalysts.