Glucuronoxylans from Moringa oleifera and other wood species obtained by alkaline extraction: Modulation of uronic acid content depending on the dosage of sodium borohydride

Abstract

Xylans are essential components of wood cell walls, composed of β-(1 → 4)-linked D-xylopyranosyl units with diverse substitutions, including 4-O-methyl-α-D-glucuronic acid. Industrial pruning of Moringa oleifera generates considerable lignocellulosic biomass that remains underutilized. In this work, glucuronoxylans (GX) were extracted from stems and branches of M. oleifera, and compared with those from Eucalyptus globulus, Cariniana legalis, and Pinus elliottii. Depectinized residues were treated with 4 M KOH under inert atmosphere, either directly or after in situ reduction with NaBH₄ at different concentrations. In the absence of delignification, NaBH₄ selectively reduced lignin–glucuronic acid esters, enabling modulation of the number of carboxyl groups in the polymer. Maximum conversion, involving 31–48 % of total 4-O-methyl-α-D-glucuronic acid, was achieved at 0.3 g NaBH₄/g of depectinized residue. Notably, this reduction occurred during extraction, offering a practical advantage over classical methods requiring prior derivatization of uronic acids. GX maintained structural integrity, and laboratory scale-up experiments confirmed the robustness of this procedure M. oleifera pruning residues represent a sustainable source of GX with tunable carboxyl functionality for further chemical modifications. This approach combines control of polymer properties with biomass valorization, prevention of waste accumulation, and contributing to circular economy.