Fractionating Wheat Straw via Phosphoric Acid with Hydrogen Peroxide Pretreatment and Structural Elucidation of the Derived Lignin

Abstract

As a newly-developed method for pretreating lignocellulosic biomass, phosphoric acid plus hydrogen peroxide (PHP) was employed as a pretreatment solvent to fractionate wheat straw. The structural properties of the derived lignin were elucidated in particular. Results indicated that 100.0 g of wheat straw (dry basis) yielded 39.7 g of cellulose-rich fraction, 4.7 g of oligosaccharides, and 4.5 g of lignin at mild conditions of 50 °C within 1.0 h. The resultant cellulose-rich fraction was highly accessible to hydrolytic enzymes with 88–96% cellulose–glucose conversion in 24 h, suggesting a great potential for producing biofuels. The derived lignin was characterized as having high purity (≤1.0% residual carbohydrates) and relatively low molecular weight (M w < 1436 g mol–1) and being abundant in carboxylic acid functional groups. According to the 31P, 13C, and 2D heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy results on the derived lignin, the degree of condensation was quite limited during PHP pretreatment, the oxidants, produced as HO+ or HO· in pretreatment, were substantially responsible for the extensive ring-opening on the aromatic substructure. The obtained results offered the technical feasibility of fractionating lignocellulosic biomass using PHP and a better understanding of the delignification mechanisms for PHP pretreatment.

Determination of hydroxyl groups in biorefinery resources via quantitative 31P NMR spectroscopy

“…A large variety of depolymerization or condensation reactions typically occur within the lignin structure under such conditions. Quantitative 31 P NMR analyses of lignins isolated from various biomass pretreatment technologies have been highlighted in recent studies 101,[108][109][110][111][112][113][114][115][116][117] . As lignin has become a key genetic engineering target for the enhancement of wood quality and biofuel production 118,119 , quantitative 31 P NMR analyses also have been carried out on transgenic lignin 97,120 .…”

Pretreatment of Wheat Straw with Phosphoric Acid and Hydrogen Peroxide to Simultaneously Facilitate Cellulose Digestibility and Modify Lignin as Adsorbents

“…Pretreatment was carried out in a 250-mL serum bottle with 8.0 g wheat straw and 80.0 g PHP mixture with different H 2 O 2 input. Then the sealed bottle was incubated in an orbital shaker at 50 • C for 3 h. After the pretreatment, the cellulose-rich fraction, oligosaccharides, and lignin were obtained via a fractionation process (see Figure S2), which has already established in our previous work [22]. The pretreatment of each condition was performed in duplicate, all results were recorded as the average of these two replicates.…”

“…Actually, the H 2 O 2 in the PHP pretreatment was responsible for releasing hydroxyl cations (HO + ) or free radicals, such as O•, OH•, which was mainly related to lignin removal by oxidation, as well as the possible modification on the recovered lignin [7,21,22]. To further elucidate the effects of H 2 O 2 input on lignin modification, the 31 P-NMR technique was applied to quantify the content of carboxylic hydroxyl groups, phenolic hydroxyl groups and aliphatic hydroxyl groups [35].…”

“…It was suggested that the ring-opening oxidation on those aromatic units was intensified with the increased of H2O2 input, and the content of carboxylic -OH was increased accordingly [21,22]. These results further explained the reason for the decreasing of typical aromatic ring absorption It was suggested that the ring-opening oxidation on those aromatic units was intensified with the increased of H 2 O 2 input, and the content of carboxylic -OH was increased accordingly [21,22]. These results further explained the reason for the decreasing of typical aromatic ring absorption bands in FT-IR spectra.…”

“…As a newly developed method, the concentrated phosphoric acid plus hydrogen peroxide (PHP) pretreatment, offers some outstanding features for pretreating lignocellulosic biomass, such as high enzymatic hydrolysis efficiency, mild conditions and wide adaptability to various raw materials (such as softwood, hardwood, agricultural residues and their mixture) [18][19][20]. When PHP pretreatment was used to fractionate lignocellulosic biomass into cellulose-rich fraction, oligosaccharides and lignin, the recovered PHP lignin (PHPL) exhibited attractive properties, i.e., high purity, low molecular weight and high content of carboxylic acid groups [21,22]. Based on these investigations, it was hypothesized that PHP can work as a bifunctional pretreatment to fractionate lignocellulose, which can potentially facilitate cellulose digestibility and modify lignin to produce adsorbents.…”