BackgroundThermochemical pretreatment of lignocellulose is crucial to bioconversion in the fields of biorefinery and biofuels. However, the enzyme inhibitors in pretreatment hydrolysate make solid substrate washing and hydrolysate detoxification indispensable prior to enzymatic hydrolysis. Sulfite pretreatment to overcome recalcitrance of lignocelluloses (SPORL) is a relatively new process, but has demonstrated robust performance for sugar and biofuel production from woody biomass in terms of yield and energy efficiency. This study demonstrated the advantage of SPORL pretreatment whereby the presentation of lignosulfonate (LS) renders the hydrolysate non-inhibitory to cellulase (Cel) due to the formation of lignosulfonate-cellulase complexes (LCCs) which can mediate the Cel adsorption between lignin and cellulose, contrary to the conventional belief that pretreatment hydrolysate inhibits the enzymatic hydrolysis unless detoxified.ResultsParticular emphasis was made on the formation mechanisms and stability phase of LCCs, the electrostatic interaction between LCCs and lignin, and the redistributed Cel adsorption between lignin and cellulose. The study found that LS, the byproduct of SPORL pretreatment, behaves as a polyelectrolyte to form LCCs with Cel by associating to the oppositely charged groups of protein. Compared to Cel, the zeta potential of LCCs is more negative and adjustable by altering the molar ratio of LS to Cel, and thereby LCCs have the ability to mitigate the nonproductive binding of Cel to lignin because of the enlarged electrostatic repulsion. Experimental results showed that the benefit from the reduced nonproductive binding outweighed the detrimental effects from the inhibitors in pretreatment hydrolysate. Specifically, the glucan conversions of solid substrate from poplar and lodgepole pine were greatly elevated by 25.9% and 31.8%, respectively, with the complete addition of the corresponding hydrolysate. This contradicts the well-acknowledged concept in the fields of biofuels and biorefinery that the pretreatment hydrolysate is inhibitory to enzymes.ConclusionsThe results reported in this study also suggest significant advantages of SPORL pretreatment in terms of water consumption and process integration, that is, it should abolish the steps of solid substrate washing and pretreatment hydrolysate detoxification for direct simultaneous saccharification and combined fermentation (SSCombF) of enzymatic and pretreatment hydrolysate, thereby facilitating bioprocess consolidation. Furthermore, this study not only has practical significance to biorefinery and bioenergy, but it also provides scientific importance to the molecular design of composite enzyme-polyelectrolyte systems, such as immobilized enzymes and enzyme activators, as well as to the design of enzyme separation processes using water-soluble polyelectrolytes.

Poplar wood was rapidly fractionated via a flow-through reaction using aqueous solutions of an acid hydrotrope (AH), p-toluenesulfonic acid (p-TsOH), at temperatures below 98 °C. 13C–1H two-dimensional nuclear magnetic resonance (NMR) spectroscopic analyses demonstrated that the AH-solubilized lignins (AHLs) from a range of fractionation conditions with yields up to approximately 80% had a very high content of β-aryl-ether linkages compared to milled wood lignin (MWL) with a low enough condensation to facilitate subsequent reductive catalytic depolymerization resulting in a lignin monomer yield of over 30%. Gel-permeation chromatographic (GPC) and differential scanning calorimetric (DSC) analyses showed that the AHLs have high molecular weights and low glass transition temperatures T g. These AHLs also have a pinkish color suitable for applications such as cosmetics and dye dispersants. AH fractionation (AHF) preserved the cellulose fraction as solid fibers also with a light pinkish color for the materials market and solubilized up to approximately 90% of xylan which can be converted to furfural using p-TsOH in the spent liquor without additional catalysts. The advantages herein are the use of one recyclable industrial chemical such as p-TsOH in an aqueous system below water boiling temperature to valorize all three major fractions of lignocelluloses in a short time frame, with very promising yields and well-preserved lignin and cellulose structure.

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Poplar wood was fractionated under a range of conditions using an inexpensive and recyclable commercial aromatic acid, p-toluenesulfonic acid (p-TsOH), as an acid hydrotrope (AH). The fractionated cellulose-rich solid fraction demonstrated excellent physical and optical properties as papermaking fibers. The dissolved xylose can be dehydrated into furfural using the p-TsOH in the spent liquor without additional catalysts. The acid hydrotrope dissolved lignin (AHL) has high β-aryl ether linkage content (∼60%) and high molecular weight (∼4000 Da), similar to that of mill wood lignin (MWL). Increasing acid hydrotrope fractionation (AHF) severity increased AHL yield, but reduced β-aryl ether bonds, decreased molecular weight and increased AHL glass transition temperature. An AHL yield of 50% can be obtained while retaining approximately half of the β-O-4 linkages, beneficial for lignin valorization as a polymer in composites as well as for production of monomers or aromatics through subsequent depolymerization.

This paper reports on an approach for separating vanillin and syringaldehyde (VSA) from oxygen delignification spent liquor using non-polar macroporous resin. The effects of temperature and pH on the adsorption isotherms were studied. The adsorption capacity and adsorption equilibrium constant were obtained using the Langmuir adsorption model at pH<4.5, where the ionization of the VSA to ionic forms was negligible. The standard enthalpy change was calculated using the van't Hoff equation and clearly showed that the adsorption is an exothermic process. The effect of pH on the adsorption isotherm is well-described by a modified Langmuir model and shows that the adsorption equilibrium constant decreases significantly with the increasing pH. The VSA adsorbed on macroporous resin was eluted by ethyl ether. The separated VSA contained 37.51% vanillin, 31.88% syringaldehyde. The recoveries of VSA were 96.2 and 94.7%, respectively.

Effective isolation of lignin containing native aryl ether linkages.

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