The synthesis, characterisation, and catalytic applications of a range of novel iron-N-heterocyclic carbenes (Fe-NHCs) immobilised on renewable resources (expanded high amylose corn starch 4a, Starbon™ 350 4b, Starbon™ 400 4c, mango peel cellulose 4d and orange peel cellulose 4e) are reported. The efficacy of the resultant heterogeneous catalysts was measured for their ability to convert: fructose to 5-(hydroxymethyl) furfural (HMF); glucose to HMF and glucose to 5-(chloromethyl)furfural (CMF).
The immobilised catalysts were prepared using a convergent strategy starting from the appropriate renewable resource, e.g., (4a-e) and 1-(3-aminopropyl) imidazole (7) to afford the precursor nitrogen heterocyclic carbene, e.g., (13a-e), which was treated with FeCl3 to furnish the desired Fe-NHC, e.g., (1a-e). Iron-loadings of 0.68 mmol g-1 for (1a), 0.31 mmol g-1 for (1b), 0.32 mmol g-1 for (1c), 0.30 mmol g-1 for (1d) and 0.52 mmol g-1 for (1e) were achieved. At 100oC and varying reaction time (10 min, 20 min, 0.5 h, 1 h, 3 h and 6 h), all catalyst types showed good performance for the dehydration of fructose to HMF: Fe-NHC expanded HACS (1a), HMF yield 86 % (t=0.5 h), TOF=206 h-1; Fe-NHC Starbon™ 350 (1b), HMF yield 81.7 % (t=0.5 h), TOF=169 h-1; Fe-NHC Starbon™ 400 (1c), HMF yield 81.0 % (t=0.35 h), TOF= 241 h-1; Fe-NHC MPC (1d), HMF yield 70.0 % (t=1 h), TOF= 79.2 h-1; Fe-NHC OPC (1e), HMF yield 71.7 % (t=0.5 h), TOF= 146 h-1. The catalysts showed very good recycling stabilities up to 5 catalytic cycles.
The fabricated catalysts were also employed in other catalytic application in important reactions including glucose dehydration to HMF, and gluose/fructose dehydration to CMF. The supported catalysts slightly effect CMF production from glucose and fructose but were ineffective towards amidation reactions of carboxylic acids and amines (as higlighted in future work).
