In the valorization of carbohydrate biomass,one of the main challenges is the efficient conversion of glucose to platform chemicals such as 5-hydroxymethylfurfural(HMF),which is generally realized by two consecutive reactions [1]: Reaction 1 is the isomerization of glucose to fructose,and Reaction 2 is the transformation of fructose to HMF.Although the catalysis of Reaction 2 by Br(o)nsted acids is efficient,detailed insight into the catalytic mechanisms is lacking.