Chemistry of Alkaline Pulping

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

“…[193] On this basis,itisareasonable assumption that the lignins following initial treatment will still exhibit good reactivity under mild conditions,a st he subunits are linked largely via these relatively weak ether linkages.H owever, during the bulk delignification stage,t he content of b-ethers drastically decreases for both the liquor-phase (Kraft) and solid-residue lignins. [190,203,204] At the final delignification point, both lignins exhibit approximately one seventh the content of b-ethers present in the initial native lignin, as inferred from analytical acidolysis data. [190] Recent HSQC NMR characterisation has confirmed that some b-O4a nd other bonding motifs (e.g., phenylcoumaran (b-5) and resinol (b-b)s tructures) are still present in aK raft lignin, albeit indeed with very low abundance.…”

“…[202] Throughout the pulping process,i ti se ssential to ensure al iquor pH value > 10, to avoid re-deposition of lignin residues onto the remaining cellulosic fibres. [203] Thed egradation and dissolution of lignin fragments from Spruce wood into the cooking liquor, as afunction of time and programmed temperature,isdisplayed in Figure 4a. [188] Thequantification of b-ether units (as inferred from analytical acidolysis) of Kraft lignin from Pinewood, both isolated from the liquor and residual in the pulp (functions of time and programmed temperature), are displayed in Figure 4b.…”

“…[196] b-Ether units predominate in native lignins.T herefore, as ubstantial level of research has been devoted to understanding the fundamental aspects of b-ether cleavage under the Kraft processing conditions.Detailed information regarding the mechanisms involved is contained in several recent book chapters. [203,204] Represented in Scheme 6isanetwork of reactions underpinning the formation of recalcitrant, highlycondensed and cross-linked (C-C) Kraft lignin from native feedstocks rich in b-ether (C-O) linkages.…”

“…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).…”

“…[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). [203] More importantly,t he formaldehyde released in the soda process can react with any free-phenolic guaiacyl unit (at its 5-position) where subsequent o-QM formation and condensation at C5 with another phenolic unit can result in additional condensation producing diphenylmethane structures (Scheme 6b,shaded in red).…”