Biphasic valorization of byproducts from biodiesel synthesis using floating photochemo-enzymatic domino catalysis

Abstract

Sustainable chemical synthesis requires a cradle-to-cradle design strategy to upcycle waste streams with optimal atom and step economy. Here, we demonstrate the full valorization of vegetable oil using a scalable two-step process. First, transesterification of triglycerides (vegetable oil) with methanol in aqueous NaOH generates biodiesel, together with glycerol and unreacted unsaturated fatty acids as byproducts. Following separation of biodiesel, glycerol and fatty acids were valorized using a biphasic domino reaction at the liquid-liquid interface with a floating photochemo-enzymatic platform. Specifically, a floatable polymeric carbon nitride/polypropylene (CNx/PP) composite photocatalyzes first the reduction of O2 to H2O2 using aqueous glycerol as the electron donor to produce glyceraldehyde and dihydroxyacetone. An immobilized lipase on a floating acrylic resin support subsequently utilizes the in situ generated aqueous H2O2 for the conversion of unsaturated fatty acids into epoxides in ethyl acetate. This novel photochemo-enzymatic domino reaction enables the full valorization of waste cooking oil at 1 kg scale, producing biodiesel (550 g), epoxidized fatty acids (438 g) and glyceraldehyde (663 g) in a 100 mL scale reactor under blue LED (λ = 450 nm, 40±4 mW cm-2 ) and simulated solar light (AM1.5G) at 40 °C generated by solar heating. A comparative life cycle assessment indicates that our integrated solar-driven process with optimized catalyst recyclability over 100 cycles has an expected 39% lower global warming potential compared to a decentralized process for conventional biodiesel, epoxy fatty acids and glyceraldehyde synthesis.