ZIF-8 as support for enhanced stability of immobilized lipase used with a thermoresponsive switchable solvent to simplify the microalgae-to-biodiesel process

Abstract

The zeolitic imidazole framework (ZIF)− 8 was tested as a support to enhance the stability of immobilized lipase. The lipase immobilized on ZIF-8, through surface attachment and encapsulation, was used for the simultaneous cell disruption and oil extraction from untreated, wet microalgal paste. The successful attachment of the enzyme to ZIF-8 was confirmed via Fourier-transform infrared spectroscopy. The attachment of the enzyme did not significantly affect the crystallinity or morphology of ZIF-8 crystals. The encapsulated lipase@ZIF-8 system showed higher stability than the adsorbed system, due to its reduced vulnerability to leaching. After five cycles, the encapsulated lipase@ZIF-8 retained 32% of its initial activity, whereas, for the adsorbed lipase@ZIF-8, it reduced to 21%. An increase in methanol amount greater than 0.2 mL was shown to have a negative effect on enzyme activity. The fatty acid methyl ester yield increased significantly with an increase in the extraction- duration (up to 3 h), after which the effect faded until 5 h, after which the equilibrium yield was reached. Changing the composition of the thermoresponsive switchable solvent (TSS) showed that a higher FAME yield could be achieved by increasing the percentages of Ionic Liquid (IL) and polypropylene glycol and reducing the water percentage. Further studies are needed to optimize the TSS composition and its effects on the process.