Candida rugosa lipase supported on biomass-based nanocellulose-silica hybrid polyethersulfone membrane for synthesis of pentyl valerate

Abstract

The expansion of oil palm plantations to meet global demands has led to new environmental challenges that rose from the production of enormous quantities of biomass. Herein, this study capitalized on extracting nanocellulose (NC) and silica (SiO2) from oil palm leaves (OPL) using a combination of chemical treatments. The nanoparticles were then used to fabricate the hybrid NC-SiO2 nanofiller for incorporation into polyethersulfone (PES) to yield the NC-SiO2-PES support for the immobilization of Candida rugosa lipase (NC-SiO2-PES/CRL). XRD, TGA-DTG and FTIR:ATR characterizations of NC and SiO2 confirmed the successful isolation of NC and SiO2 from lignocellulosic resources. A 0.02 cm membrane size with 5% (w/v) of NC-SiO2 without PVP K30 was optimal for membrane fabrication. Modification of NC-SiO2-PES was conducted using 3-(aminopropyl)triethoxysilane (APTES) followed by activation with glutaraldehyde that gave the highest conversion of pentyl valerate (91.3%, p < 0.05) compared to Glut-NC-SiO2-PES (73.9%, p < 0.05) in 3 h. The optimized Taguchi Design-assisted immobilization of CRL onto NC-SiO2-PES membrane (5% glutaraldehyde, 4 h of immobilization, 20 mg/mL CRL concentration, 40 °C and pH 5) gave a 90% yield of PeVa in 3 h. Characterization of NC-SiO2-PES/CRL biocatalyst using FTIR-ATR, XRD, TGA-DTG, FESEM-EDX, TEM, AFM and Raman spectroscopy revealed that the CRL molecules were successfully bound to the surface of the NC-SiO2-PES membrane via imine bonds formed through a Schiff base mechanism. The results indicated that NC formed intermolecular hydrogen bonds with SiO2, while OH groups from the resultant NC-SiO2 forged hydrogen bonds with S=O of PES. The thermal stability of NC-SiO2-PES/CRL was ~30% higher over the free CRL, with reusability for up to 14 successive esterification cycles. NC-SiO2-PES/CRL also exhibited extended operational stability, with a robust half-life of ~120 h, excellent storage stability at 4 °C, and the absence of leached protein after 60 min of agitation. Kinetics evaluation showed that the NC-SiO2-PES/CRL-catalyzed synthesis of PeVa followed the ping-pong bi-bi mechanism (Vmax of 0.57 mM min-1) with pentanol inhibition (Ki,B 78.49 mM). Meanwhile, the Michaelis-Menten constants for substrates, valeric acid (Km,A) and pentanol (Km,B) were 67.97 mM and 43.53 mM, respectively. The higher values of ?H°d, ?G°d, t1/2 and activation energy of enzyme denaturation (Ed) conveyed that the NC-SiO2-PES improved the thermal stability of the CRL and the process followed first-order kinetics (R2 > 0.95). The activation energy (Ea) and activation energy for thermal denaturation (Ed) for the NC-SiO2-PES/CRL was 6.49 kJ mol-1 and 96.8 kJ mol-1, respectively. The NC-SiO2-PES/CRL's ability to be regenerated chemically and ultrasonically, and reused without significant loss in enzyme activity denotes its potential cost-saving for the production of PeVa. The FTIR, 1H-NMR, gas chromatography-mass spectrometry ([M]+ m/z 130, C10H20O2) verified the enzymatically produced PeVa. The overall findings invariably envisaged the biocompatibility of NC and SiO2 derived from OPL as a suitable nano-filler to prepare the NC-SiO2-PES composite for CRL immobilization. Therefore, the NC-SiO2-PES/CRLs are a potential immobilized biocatalyst to expedite the synthesis of high yields of PeVa.