Catalytic Hydrolysis of Caragana korshinskii to Valuable Bio-Oils over a Magnetic Iron Niobate: Combined with Structural Feature Evaluation

Abstract

Caragana korshinskii (Ck) is a typical tillering deciduous shrub whose branches have certain medicinal value. In this work, a magnetic iron niobate (FeNb₂O₆) nanosphere was prepared and used for the catalytic hydrolysis of Ck to valuable bio-oils. FeNb₂O₆ has good >C–O- cracking activity and hydrothermal stability. Under optimal conditions (Ck/FeNb₂O₆ = 5/1, 1 MPa H₂, 300 °C, and 60 min), the bio-oil yield is as high as 40.6 wt %. Structural features of Ck, derived residue (R_CH), and bio-oil (SP_EA) were further evaluated. Solid-state ¹³C NMR, XPS, TG/DTG, and FTIR analyses show that C_aliphatic moieties dominate Ck with abundant >C–O- groups. Core units of most aromatic clusters are polysubstituted benzene rings. FeNb₂O₆ promotes the cleavage of −COO-, >C–O-, and >N-X bridged bonds in Ck. Resulting SP_EA is rich in aliphatic and >C═O/>C–O- moieties, while R_CH is characterized by lignin. GC/MS and ¹H NMR analyses further show that >C_alOH, >C_ar-O-, and >C═O are 3 core classes in SP_EA, and alkanols, cyclanols, arenols, alkoxy-substituted phenols, and ketones dominate. Aromatics are mostly ortho-para substituted and mainly attributed to the release of inherent guaiacyl/syringyl units. On this basis, the structure information with higher accuracy, including four basic units, bridged bonds, and aromatic fragments, was obtained by GC/MS and 2D NMR matching. Bridged bonds involve β-O-4, α,β-diaryl ethers, β-5, β–β, and carbohydrates, while aromatic fragments refer to guaiacyl/syringyl, ferulate, p-hydroxybenzoate, and p-coumarate units as well as the cinnamyl alcohol end-group.