Decyl esters production from soybean-based oils catalyzed by lipase immobilized on differently functionalized rice husk silica and their characterization as potential biolubricants

Abstract

This study aimed the enzymatic decyl esters production by hydroesterification, a two-step process consisting of hydrolysis of refined soybean (RSBO) or used soybean cooking (USCO) oils to produce free fatty acids (FFA) and further esterification of purified FFA. Using free lipase from Candida rugosa (CRL), about 98% hydrolyses for both oils have been observed after 180 min of reaction using a CRL loading of 50 U g^-1 of reaction mixture, 40 °C, and a mechanical stirring of 1500 rpm. FFA esterification with decanol in solvent-free systems was performed using lipase from Thermomyces lanuginosus (TLL) immobilized by physical adsorption on silica particles extracted from rice husk, an agricultural waste. For such purpose, non-functionalized (SiO₂) or functionalized rice husk silica bearing octyl (Octyl-SiO₂) or phenyl (Phe-SiO₂) groups have been used as immobilization supports. Protein amounts between 22 and 28 mg g^-1 of support were observed. When used in the esterification, they enabled a FFA conversion of 81.3-87.6% after 90-300 min of reaction. Lipozyme TL IM, a commercial immobilized TLL, exhibited similar performance compared to TLL-Octyl-SiO₂ (FFA conversion ≈90% after 90-120 min of reaction). However, high operational stability after fifteen successive esterification batches was observed only for TLL immobilized on Octyl-SiO₂ (activity retention of ≈90% using both FFA sources). The produced decyl esters presented good characteristics as potential biolubricants according to standard methods (ASTM) and thermal analysis.

Keywords: Biolubricants; Immobilized lipase; Physicochemical properties; Soybean oils.