Synthesis of reusable lipases by immobilization on hexagonal mesoporous silica and encapsulation in calcium alginate: transesterification in non-aqueous medium

Abstract

Hexagonal mesoporous silica (HMS) was used for pre-immobilization of Candida antarctica lipase B (CAL B) and the effect of calcium alginate encapsulation was studied. Pseudomonas cepacia lipase (PSL), Candida rugosa lipase (CRL) and porcine pancreatic lipase (PPL) were also studied, amongst which CAL B was found to be the most active for transesterification reaction. All lipases were employed as biocatalysts in four different physical forms, such as unsupported, immobilized on HMS, encapsulated in calcium alginate (CA) and pre-immobilized on HMS and encapsulated using CA. Pre-immobilization of CAL B on hexagonal mesoporous silica (HMS) was carried out by simple physical adsorption with an immobilization yield of 62% and the maximum loading of the enzyme was 100 mg per gram of support. The recovered activity which was retained on the support was found to be 26%. The pre-immobilized enzyme when encapsulated in calcium alginate beads yields a highly reusable biocatalyst with no leaching even after the fourth reuse. This novel way of preparation of a biocatalyst overcomes the problem of leaching of the enzyme from mesoporous support by avoiding a direct shear involved within the stirred reactor. The activities of all lipases in different physical forms were evaluated in the transesterification of p-chlorobenzyl alcohol with vinyl acetate to give p-chlorobenzyl acetate at 30 °C. A conversion of 68% with 100% selectivity for p-chlorobenzyl acetate was obtained at 30 °C in 120 min, with CAL B being pre-immobilized on HMS and encapsulated using CA (CAL B/HMS/Encap) using equimolar concentrations of reactants and 1,4-dioxane as solvent. CAL B/HMS/Encap showed excellent reusability with a decrease of only 4% in the overall conversion of the transesterification reaction even after the fourth reuse.