Structural heterogeneity of castor tissue lignins and their targeted conversion into sustainable aviation fuel precursors

Abstract

Castor is an important non-edible oilseed crop worldwide, and its residual biomass represents an ideal feedstock for biorefineries. However, the structural characteristics of its lignin and lignin-carbohydrate complex (LCC), along with their valorization potential for sustainable aviation fuel (SAF) precursors, remain poorly understood. Herein, enzymatic mild acidolysis lignin (EMAL) and LCC were isolated from castor endocarp, epicarp, and stalk, followed by systematic structural characterization. The endocarp exhibited the highest lignin content (66.04 wt%), with its EMAL predominantly composed of catechyl units and benzodioxane linkages, which accounted for 87% of the detectable bonds. In contrast, the EMALs derived from the epicarp and stalk were rich in guaiacyl and syringyl units, primarily linked via β-O-4′ bonds.Catalytic hydrogenolysis of the isolated EMALs afforded distinct phenolic monomers, with total yields ranging from 36.39 wt% to 45.32 wt%. Subsequent hydrodeoxygenation of these depolymerized lignin oils produced liquid cyclic hydrocarbons, achieving yields of 22.88 wt% for the endocarp, 33.96 wt% for the epicarp, and 34.12 wt% for the stalk. Notably, the carbon number distributions of all upgraded products fell precisely within the target C9-C16 range, meeting the technical specifications required for commercial SAFs. This work provides valuable insights into the fundamental structural properties of EMAL and LCC across different castor tissues and highlights their significant potential for targeted conversion into advanced aviation biofuels.