FF Formation Pathway. Two reaction pathways to form FF from either HEH or 3-hexulose, respectively, were identified from the reaction exploration with similar overall barriers. The HEH pathway to FF diverges from the analogous pathway to HMF at species 7-3, where a formaldehyde elimination reaction occurs (retro-aldol reaction) to yield species 8-1 instead of hydrogen migration in the case of the HMF pathway. Intermediate 8-1 can go through either a three-step reaction mechanism involving a proton transfer and [1,5] bond shift (Fig. ), hydrogen migration rearrangement and double water-catalyzed ring closure, or a one-step single water-catalyzed cyclization to form intermediate 9-3 (similar to Fig. ). Similar to HMF formation, double water-catalyzed dehydration reactions are the last step in all pathways to form FF (9-3→FF in Fig. ). For the alternate hexulose pathway, after the elimination of formaldehyde from 3-hexulose, the remaining steps have the same reaction mechanism as the pathway forming HMF. The rate-limiting step of these two pathways are cyclization and formaldehyde elimination with overall barriers of 35.31 and 35.36 kcal/mol, respectively.