Complementing Vanillin and Cellulose Production by Oxidation of Lignocellulose with Stirring Control

Lignin transformation to high-value chemicals is key for forthcoming biorefineries. Here, we report efficient delignification of pine wood by oxidative biorefining in aqueous alkali, producing both vanillin and cellulose as valuable end products. Under optimal conditions, viz., 400 rpm, 160 degrees C, 1 h, 7.5 wt % NaOH, and 1 MPa O-2, more than 90% of lignin is converted and fractionated into monophenolics (29 wt % on Kraft lignin basis and vanillin yield is 21.1 wt %) and small oligophenolics (56.7 wt %, average M-w between 300 and 600 Da), next to a pool of small carboxylic acids. Depending on the conditions, a substantial amount of white residue can be formed as a coproduct, comprising 45% yield of 95.5% pure fibrous crystalline cellulose I, or a less pure cellulose residue can be obtained, which is highly reactive toward levulinic and formic acid formation using acid catalysis. Liquor refining by liquid-liquid extraction facilitates thorough identification of the main products by gas chromatography-mass spectrometry (MS), heteronuclear single quantum coherence NMR, electrospray ionization quadrupole time-of-flight mass spectrometry, and gel permeation chromatography. The mass balance shows close to 70% carbon efficiency of biomass conversion into useful products. Surprisingly, agitation speed, in addition to base concentration, temperature, and reaction time, is a crucial parameter to overcome deeper oxidation/condensation of soluble lignin fragments and to avoid substantial cellulose degradation. Similar reactions with other feedstock such as eucalyptus (hardwood), grasses, and a bagasse waste stream demonstrate the feasibility of the reported oxidative catalytic fractionation and the strong dependency of the product distribution on biomass variability within the different fractions.