Oil palm leaves ash silica-magnetite-candida rugosa lipase nanoconjugates for synthesis of butyl butyrate

Abstract

Although modern technologies have successfully converted a certain percentage of the oil palm biomass into useful bio-products, potentials of the largest oil palm biomass, that is oil palm fronds, have not been fully explored. In this study, a comprehensive physicochemical characterization of the Malaysian oil palm leaves (OPL) was carried out to establish suitability of its composition for industrial applications. Ultimate analysis revealed that the untreated OPL contained carbon (46.98 %), hydrogen (6.50 %), nitrogen (1.81 %) and sulfur (0.15 %) with a moderately high calorific value of 19.21 MJ/kg. Thermal gravimetric analysis indicated that OPL is a lignocellulosic material whereas X-ray fluorescence spectroscopy revealed Si (95.30 %) as the predominant element for acid treated OPL sample. Nanosilica extracted from OPL ash was coated on magnetite and was modified with APTES and glutaraldehyde. Suitability of protocol to immobilize Candida rugosa lipase (CRL) onto modified OPL-silica were assessed at different concentrations of glutaraldehyde and CRL solutions, as well as time and temperature. Data on surface topography and morphology obtained by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetry analysis-differential thermogravimetry (TGA-DTG), atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) showed that CRL was present on the surface of the support (Gl-A-SiO2-MNPs) as CRL/Gl-A-SiO2-MNPs. Immobilization parameters showed that approximately 80 % of CRL initially available was covalently bound onto the Gl-A-SiO2-MNP. Maximum protein loading and specific activity of 67.50 mg/g and 320.80 ± 0.42 U/g were obtained, respectively. Optimal conditions that gave the highest yield of butyl butyrate (94 %) by OVAT and Box-Behnken design were 3.50 mg/mL protein loading, incubated for 3 h at 45 ? using 1-butanol:n-butyric acid ratio 2:1. CRL/Gl-A-SiO2-MNPs showed an extended operational stability, attaining 50 % of its initial activity after 17 of consecutive esterification cycles. CRL/Gl-A-SiO2-MNPs catalyzed the esterification synthesis to produce butyl butyrate according to the Ping Pong bi-bi mechanism with inhibition by both substrates.