Importance of the synergistic effects between cobalt sulfate and tetrahydrofuran for selective production of 5-hydroxymethylfurfural from carbohydrates

In this study, an effective catalytic system (CoSO₄·7H₂O/THF) for selective conversion of fructose to 5-hydroxymethylfurfural (HMF; yield: 88%) was developed. The synergistic effects among Co²⁺, SO₄^2-, crystal water and tetrahydrofuran (THF) were crucial for achieving selective dehydration of fructose to HMF. Co²⁺ worked as a Lewis acid for catalyzing mainly dehydration of fructose to HMF but not the further decomposition of HMF to levulinic acid. THF could help to retain HMF while CoSO₄ could coordinate with HMF, enhancing the thermal stability of HMF in THF. The crystal water in cobalt sulfate could help to coordinate with fructose, which facilitated the conversion of fructose via dehydration reactions. The CoSO₄·7H₂O/THF catalytic system could also catalyze the conversion of inulin and cellulose into HMF. The main advantages of the CoSO₄·7H₂O/THF catalytic system are the low cost, the easy recycling of the CoSO₄·7H₂O catalyst and the easy separation of HMF from volatile THF.