Reductive Degradation of Lignin and Model Compounds by Hydrosilanes

Abstract

The exploitation of lignin, the second most abundant naturally occurring polymer on earth, has been hampered by its network structure, which makes it difficult to process. Hydrosilanes have previously been shown to convert aryl ethers to hydrolyzable silyl ethers in the presence of B­(C6F5)3. We demonstrate that the process is general and can be used to convert model lignin compounds to both aryl silyl ethers and alkanes. The relative reactivity of functional groups on model lignin compounds was found to be phenol > primary alcohol > methoxybenzene > alkyl silyl ethers. The process thus leads to cleavage of β-O-4, α-O-4, and methoxybenzene groups with concomitant silylation of phenolic and secondary alcohol groups. At longer time points provided sufficient silane was present, the full reduction of primary and secondary alcohols to alkyl groups was observed. Softwood lignin itself could only be partially solubilized (∼30%) even using excess hydrosilane and high catalyst loadings; the products were not characterized in detail. The lack of further degradation was attributed to its highly branched network structure containing 5-5, β-5, 4-O-5, and other linkages derived from coniferyl alcohol monomers that are not susceptible to reductive silylation. By contrast, over 95% of hardwood lignin was efficiently reduced/degraded into organosoluble products by the monofunctional hydrosilane HMe2SiOSiMe3 over a few hours at 50 °C. The molecular weight of the silylated products was consistent with oligomeric structures comprised of 3–8 linked aryl groups. This process holds promise to increase the accessibility to value-added products using lignin as a starting material.

Convergent reductive depolymerization of wood lignin to isolated phenol derivatives by metal-free catalytic hydrosilylation

“…The sulfite and Kraft processes target the production of high quality cellulose for the pulp and paper industry and the resulting Kraft lignin 62 and lignosulfonates 63 are generated as byproducts. 65 In contrast, extraction methods utilizing organic solvents (organosolv treatment) operate under milder conditions and they are known to preserve the native structure of lignin by reducing C-C coupling reactions. Nevertheless, because lignin is obtained under harsh and/or acidic conditions, the resulting polymeric structure is significantly altered from native lignin and features an increased proportion of C-C based linkages.…”

Foamed lignin–silicone bio-composites by extrusion and then compression molding

“…1, ESI †) were produced at either lower (60°C) or higher (120°C) molding temperatures with identical formulations (F-4A and C, BCF = 2050 ppm). 48 Post-curing out of the mold. sized bubbles, while at higher temperatures, uncontrolled rapid foaming occurred at the start of the reaction, again leading to large voids.…”

“…47,48 The process leads to the conversion of phenol, arylmethoxy groups, and other ethers into alkyl groups or silyl ethers (Scheme 1A). 47,48 The process leads to the conversion of phenol, arylmethoxy groups, and other ethers into alkyl groups or silyl ethers (Scheme 1A).…”

Catalytic Conversion of Fructose and 5-Hydroxymethylfurfural into 2,5-Furandicarboxylic Acid over a Recyclable Fe3O4–CoOx Magnetite Nanocatalyst

“…In this context, magnetite nanoparticles have been attracted much attention for various organic reactions. 12,13 Biomass is the only carbon containing renewable resource. More importantly, their unique paramagnetic properties and inherent insolubility allow them to be efficiently separated from the reaction mixtures by a magnet.…”