Application of a lipase-immobilized silica monolith bioreactor to the production of fatty acid methyl esters

We developed a highly efficient bioreactor loaded with a lipase-immobilized non-shrinkable silica monolith by adopting a two-step sol–gel method, i.e., preparing a methyltrimethoxysilane (MTMS)-based silica monolith followed by coating of the latter with more hydrophobic alkyl-substituted silicates that entrapped lipase. We applied this type of bioreactor to the production of fatty acid methyl esters through methanolysis of rapeseed oil in n-hexane by Rhizopus oryzae lipase. As the result of screening alkyltrimethoxysilanes mixed with tetramethoxysilane (TMOS) during sol–gel coating, propyltrimethoxysilane (PTMS) gave the best performance, and the lipase immobilized therein exhibited ca. 10 times higher activity than non-immobilized lipase powder. The amount of the PTMS-based silicates with which the MTMS-based silica monolith was coated was optimized by adjusting the molar ratio of silanes (mixture of PTMS and TMOS at 4:1) to 100 mol of water. Conversion rate was highest at the molar ratio of 0.4 mol silanes to 100 mol of water, which was ca. 10 times higher than that with lipase deposited on the MTMS-based silica monolith. Conversion rate was approximately 1.5 times higher in the flow-through monolith bioreactor than in the batch slurry reactor.