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

Abstract

Metal (M) exchanged dodecatungstophosphoric acid (DTP) encapsulated inside ZIF-8 framework were synthesized by an in situ method to produce a variety of active catalysts. Thus, M-DTP@ZIF-8 (M = Co, Zr, Al and Fe) were used for the formation of furfuryl acetate from the esterification of furfuryl alcohol and acetic acid. The highest activity of the catalyst was found for Fe-DTP@ZIF-8, which was also the catalyst with the highest number of acidic sites. DTP 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. Fe-DTP@ZIF-8, the most active catalyst, possessed a very high surface area (1114 m² g⁻¹) and the highest acidity (0.61 mmol g⁻¹). Fresh and spent catalyst was characterized using NH₃-TPD, powder XRD, DRIFTS, SEM, HRTEM and TGA. A kinetic model was developed based on the LHHW (Langmuir–Hinshelwood–Hougen–Watson) mechanism and activation energy was calculated for both the steps as 38.6 kJ mol⁻¹ and 65.2 kJ mol⁻¹, which proved that the esterification reaction was much faster than the self-etherification of furfuryl alcohol.