Lignin is renewable and the most abundant aromatic sources that can be used for extensive chemicals and materials. Although approximately 50 million tons of lignin are produced annually as a...

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γ-valerolactone (GVL)/H2O/acid solvent mixtures has been used in chemical pretreatment of lignocellulosic biomass, it was claimed that GVL lignins were structurally close to proto (native) lignins, or having low molecular weight with narrow polydispersity, however, the structural changes of GVL lignins have not been investigated. In this study, β-O-4 (β-aryl ether, GG), β-5 (phenylcoumaran), and β-β (resinol) lignin model compounds were treated by an acidic GVL-H2O solvent system, a promising pretreatment of lignocellulose for biomass utilization, to investigate the structural changes possibly related to the lignin involved. NMR characterization of the products isolated from the treated GG indicated that a phenyl dihydrobenzofuran, having typical C-H correlations at δC/δH 50.74/4.50 and 93.49/4.60 ppm in its HSQC spectrum, was produced from GG. In the pretreatment, the released formaldehyde from GG reacted fast with GG to form a novel 1,3-dioxane intermediate whose characteristic HSQC signals were: δC/δH 94.15–94.48/4.81–5.18 ppm and 80.82–83.34/4.50–4.94 ppm. The β-5 model, dihydrodehydrodiconiferyl alcohol, was converted into phenylcoumarone and stilbene having benzaldehyde that resulted from the allyl alcohol side chain. The β-β model, syringaresinol, was isomerized to form a mixture of syringaresinol, epi-, and dia-syringaresinol although being degraded slightly.

Phenolation is one of the most efficient ways to improve lignin's reactivity by increasing phenolic hydroxyl (Ph-OH) contents and active sites. However, due to its complex structural nature, the lignin structure significantly impacts the phenolation and its following nanoparticle fabrication. In this study, phenolation of cellulolytic enzyme lignin (CEL) and kraft lignin (KL) of Simao pine was conducted under appropriate acidcatalyzed conditions and the resultant phenolated lignins were comprehensively characterized. The main structural difference between KL and CEL arises from the content of β-aryl ether bonds, leading to varied increments of Ph-OH contents in phenolation. Size-controlled lignin nanoparticles (LNPs) were successfully prepared from phenolated lignins with lower molecular weight, lower aliphatic-OH content, and higher level of Ph-OH groups. The average diameters and surface charges of LNPs varied from 60 to 120 nm and −30 to −55 mV, respectively, which highly depended on the phenolation degree of lignin, especially the content of Ph-OH groups. It indicates that the structural features of phenolated lignin could facilitate LNP fabrication with tunable sizes. This study reveals the structural influences on lignin phenolation and its nanoparticle fabrication, providing new insights into the size-controlled design, preparation, and application of LNPs.

Phenolation is a commonly used method to improve the reactivity of lignin for various applications. In this study, resinol lignin models (syringaresinol and pinoresinol) and eucalyptus alkali lignin were treated under acid-catalyzed phenolation conditions to investigate the products derived from resinol (ββ) structures of lignins. The phenolation products were characterized by means of GC-MS and NMR spectroscopy following separation using flash chromatography and thin-layer chromatography. A series of new naphthalene products were identified from phenolation of syringaresinol, and the corresponding guaiacyl analogs were also identified by GC-MS. The C1À Cα bond of these resinol compounds was cleaved to release syringol or guaiacol during phenolation. In addition, diphenylmethane products formed from phenol or phenol and syringol/ guaiacol were found in the phenolation products. Comparatively, more naphthalene products were obtained by phenolation from syringaresinol than those obtained from pinoresinol. HSQC NMR characterization of the phenolated alkali lignin revealed that naphthalene structures formed in the phenolated lignin.