Lipase-catalyzed synthesis of sugar alcohol esters as biobased surfactants

Abstract

Saccharide-based esters have been widely utilized as surfactants. The conventional production process involved high temperature and homogenous alkaline catalysts in hazardous organic solvents. This process produced mixture of discolored products that required complicated purification steps to obtain high quality products. Not to mention that the hazardous organic solvents pose a threat to the environment. Enzyme-catalyzed process offers a greener approach to produce surfactants. In this present work, we investigated lipase-catalyzed synthesis of sorbitol esters, a type of saccharide-based surfactants that can be derived from inexpensive renewable agricultural sources, such as carbohydrate and palm oil. This work was divided to four main phases. In phase 1, various reaction parameters affecting lipase-catalyzed synthesis of sorbitol esters were investigated including different types of solvents and lipases, substrates mole ratio, molecular sieves and lipase concentration, reaction temperature and time and also different fatty acid chain length. The highest conversion achieved was 76% within 24 h under optimal conditions; sorbitol (0.4 M), fatty acid (0.8 M), 20 wt.% Lipozyme® TL IM (immobilized Thermomyces lanuginose) in 100 ml tert-butanol at 55°C for 24 h in the presence of 25 wt.% 3Å molecular sieves. The focus of phase 2 was the optimization of process parameters with the aim to increase reaction conversion by employing two important strategies; high speed homogenization to improve solubility of sorbitol and addition of molecular sieves later in the reaction for a more efficient water controlled of the system. These two measures significantly improved reaction conversion compared to the results obtained in previous phase. Reactions conducted with caprylic, capric, lauric and myristic acids resulted in 100% conversion at different durations: 24, 26, 28 and 32 h, respectively. Whereas palmitic, stearic and oleic acids gave more than 87% conversion over 34 h of reaction. In phase 3, characterization of products using mass spectrometry and nuclear magnetic resonance found that the biocatalyst (Lipozyme® TL IM) was highly regioselective, esterifying exclusively at sorbitol’s primary hydroxyl groups, producing 1- and 6-sorbitol monoesters. Lipozyme® TL IM also exhibited substrate selectivity towards shorter chain acyl donors and showed good reusability performance. In phase 4, the surfactant properties of the synthesized sorbitol monoesters were evaluated. The sorbitol monoesters showed good surface tension and emulsification properties, comparable to several commercial products. The biocatalytic process reported in this work is a one-step process to produce biobased surfactants that does not involve the use of toxic or expensive solvents and does not require pre-derivatization of the substrate molecules.