A Novel Iron-NHC Catalyst Supported on Expanded Starch: Synthesis, Characterization and Application

The synthesis and characterization of a novel iron-N-heterocyclic carbene (iron-NHC) supported on expanded high amylose corn starch (HACS), using a convergent strategy, and its application as efficient catalyst for fructose to HMF conversion is reported. The NHC ligand was prepared via a multi-step approach and tethered on to suitably derivatized (succinimidyl carbonate, DS = 0.33±0.11) expanded HACS in good yield (61.5%). The synthesis was carried out using green(er) solvents such as propylene carbonate and CPME in preference of DMF and CH2Cl2 where appropriate. An iron-loading of 0.26 mmol/g was achieved. Present of iron was confirmed qualitatively by colour change and quantitatively by ICP in addition to complementary XPS, TGA, STA and TEM analysis. However, it is assumed that all iron detected is complexed to the NHC ligand and not trapped within the mesoporous structure of the starch support.
After modification, BET surface area and pore volume dropped from 186.65 m2/g and 0.91 cm3/g for expanded starch to 135.49 m2/g and 0.60 cm3/g for the final Fe-NHC catalyst. The drop in pore volume may be due to possible blocking and filling of the porous structure by the ligand and iron.
The Fe-NHC was proved to be heterogeneous as no discernible iron leaching was observed and it showed good performance for the dehydration of fructose to 5hydroxymethyl furfural (HMF) when the reaction was explored at 100 ◦C and varying time (0.5 h, 1 h, 3 h and 6 h). Best yield (63.54%) was achieved at t=1 h with a fructose to HMF conversion of 75.83% and HMF selectivity of 83.79%. Interestingly, under nitrogen flow, poor conversion was reported.
Catalyst re-usability investigation showed that it can be re-used for up to 4 times without significant loss in performance. Comparison of catalytic activity to other heterogeneous catalysts, e.g. Amberlyst-15, Montmorillonite K-10 and ZSM-30 reported relatively similar behaviour.