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4 h (heating period is not included), the autoclave was cooled to room temperature and carefully depressurized to normal pressure. Then the products were analyzed by GC. 2.2 GC Measurements The attempt to analyze all these products using HPLC directly failed, as it was difficult to separate all compounds perfectly. GC was used to determine products. Gas chromatography measurements were conducted on Agilent 7890A GC with autosampler and a flame ionization detector. DB-17 capillary column was used for separation of products. 1,2,4,5-tetramethyl- benzene (TMB) was used as an internal standard. The amounts of maleic anhydride (MA), DFF and residual HMF were determined by GC with internal standard method. They were identified by GC-MS and by comparison with authentic compounds. The HMF conversion (mol%) and the yield (mol%) of main furan products were calculated using the amount of initial HMF as the denominator and the absolute amount of DFF or MA as the numerator, as shown below:

6 Notes: Scheme S1: Steps for the determination of DFF and the residual HMF Scheme S2: Steps for the determination of MA MA is reactive with water. Maleic acid indeed existed in the products (Fig. S13). However, there is no signal for maleic acid on GC. In order to study the oxidation conveniently and accurately, we transform the maleic acid to maleic anhydride by dehydration with excess acetic anhydride before GC determination. This pretreatment was verified using pure maleic acid experimentally. The procedure is as follows: After cooled and depressurized, all the crude mixture was transferred to

50 ml round-bottomed flask with condenser tubes followed by

5 ml acetic anhydride. The mixture was heated to reflux for

2 h under stirring (Scheme S2). Therefore two parallel experiments were performed to get data. The yield of DFF was obtained directly by GC measurement without dehydration with acetic anhydride;

and the residual HMF was also determined without dehydration. The total yield of MA including maleic acid was obtained by GC measurement after dehydration with acetic anhydride (Fig. S10);

The yield of maleic acid was calculated as the MA yield diffe........