Reversible lignin stabilization in traditional aqueous pulping conditions using boric acid

Lignin’s uncontrolled condensation during wood pulping in aqueous alkaline conditions has long limited the valorization of technical lignin’s aromatic functionality for chemical production. Several “lignin-first” processes have been proposed to chemically stabilize lignin during biomass fractionation, allowing its valorization to mono- or bisphenols or as high-performance adhesives.1–3 However, all these methods are orthogonal to traditional pulping, starting with their systematic use of organic solvents to solubilize lignin and facilitate stabilization. Here, we used boric and boronic acid chemistry to stabilize lignin in traditional aqueous alkaline pulping conditions (130-180°C, pH 13-14). Notably, boric acid (BA) reversibly formed cyclic esters with lignin’s β–O–4 hydroxyl groups in water, enhancing lignin solubility beyond Kraft lignin suggesting that BA could partially replace Na2S used in the Kraft process. Simple acidification led to near-complete (>99.7%) recovery of BA and produced uncondensed lignin that could be used directly as a wood adhesive, or hydrogenolyzed to monophenolics at yields ~3-fold those of lignin extracted without BA. The process recovered 95% glucan as cellulosic fibers with comparable properties to Kraft pulp, and 80-90% xylan with >96% purity. BA use thus offers a practical route to directly integrate lignin-first chemistry and xylan recovery into existing pulp mills.