Wood Based Lignin Reactions Important to the Biorefinery and Pulp and Paper Industries

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

“…Thes econd major depolymerising reaction, which is exclusive to the Kraft process, involves the trapping of quinone methide (QM) intermediates in an essential step in the cleavage of free-phenolic b-aryl ethers,i ncluding those that get produced by these nonphenolic b-ether cleavage reactions (Scheme 6b). [203,205] Under the conditions of the Kraft process (HS À /OH À ), the QM undergoes addition of nucleophilic HS À ,followed by the elimination of ap henolate species via anchimeric assistance and formation of at hioepoxide. [203] This is the particular reaction that explains the improvement seen upon introduction of sulfide (and the HS À produced during the process) to the earlier soda (NaOH-only) process.I ns oda pulping,t he retro-aldol elimination of g-CH 2 OH as formaldehyde from the QM occurs more frequently.Such atransformation leads to styryl ether (sometimes called by the less specific term, vinyl ether) structures that are quite stable under alkaline conditions (but are prone to hydrolysis under acidic conditions).…”

“…[178] Under the harsh conditions of the Kraft process,the freephenolic guaiacyl units are prone to undergo multifarious repolymerisation processes (Scheme 6b,r egion shaded in red). [203,205] Accordingly,t here is an oticeable increase in molecular weight of lignin fragments in the liquor (i.e., those peeled from the lignocellulosic matrix) measured at the three stages (initial, bulk and final) of delignification (Figures 4and 5). [191] As noted above,f ormaldehyde,l iberated by the elimination of g-CH 2 OH groups,p lays ac ritical role in the repolymerisation process (see Section 3.4.4) but this formaldehyde elimination is markedly less problematic in Kraft pulping than in the soda process.…”

“…In fact, the thermodynamic costs associated with breaking stable CÀC bonds will invariably be 30-60 kcal mol À1 higher (Scheme 6b) than those required for the cleavage of b-ether linkages in native lignins.T hereby,i ti sh ighly desirable to "tune" the Kraft process in order to avoid recondensation of the lignin fragments.Unfortunately,this is not atrivial task, due to the multitude and chemical variety of repolymerisation channels. [205] Alternatively,c ontinuous extraction of monomeric and oligomeric species from the black liquor could be an effective strategy,a nd would alleviate problems associated both with the self-condensation of lignin and the condensation of lignin with hemicellulosic sugars. [208] Ther emoval of soluble carbohydrates is also desirable so as to avoid problems in subsequent downstream processing.…”

Ethyl Levulinate Obtained from Lignocellulosic Waste Material with Previous Delignification by Ultrasonic-Assisted Technique

“…Experiments were performed changing the pH from Figure 4 and Figure 5). This cleavage reaction has been described by several authors [25]. Another proposed route also involves the breakdown of the β-O-4 bonds, generating formaldehyde from the γ position and another type enol-ether structure, is described in Figure 4.…”

“…It has been reported that these reactions occur in acid medium at elevated temperatures. Because of this and the type of acid used that influences the formation of the compounds formed during the process [25] is likely that low yields are observed in the delignification process in general. …”