Novelty of iron exchanged heteropolyacid-encapsulated inside ZIF-8 as active and superior catalyst in esterification of furfuryl alcohol and acetic acid

Metal exchanged dodeca-tungstophosphoric acid (DTP) encapsulated inside the ZIF-8 (M-DTP@ZIF-8 where M = Co, Zr, Al and Fe) framework synthesized by an in situ method produced catalysts active for the formation of furfuryl acetate from the esterification of furfuryl alcohol and acetic acid. The highest activity of the catalyst was found for the Fe-DTP@ZIF-8, which was also the catalyst with the highest number of acidic sites. Dodeca-tungstophosphoric acid showed retention of the Keggin structure after exchanging protons with the metal ions and upon encapsulation of ions, the rhombic structure of ZIF-8 was also retained. The Fe-DTP@ZIF-8, the most active catalyst, possessed a very high surface area (1114 m2/g) and the highest acidity (0.61 mmol/g). Fresh and spent catalyst was characterized using NH3-TPD, powder XRD, DRIFTS-, SEM, HRTEM and TGA. A kinetic model was developed based on LHHW (Langmuir-Hinshelwood–Hougen-Watson) mechanism and activation energy was calculated for both the steps as 38.6 kJ/mol and 65.2 kJ/mol which proved that the esterification reaction was much faster than the self-etherification of furfuryl alcohol.